Integrated optical element, optical pickup, and optical disk device

ABSTRACT

A hologram  48  as optical path branching element has two hologram areas  48   b  and  48   c  for diffracting a return light beam into different directions, and a boundary  48   a  between them is inclined at a predetermined angle with respect to the direction corresponding to the direction of radius of the optical disc  11 . The light diffracted by the one hologram area  48   b  is received by light-receiving sections A 1  and A 2  of a photodetector IC  44 , and the light diffracted by the other hologram area  48   c  is received by light-receiving sections A 3  and A 4 . With the above-described structure, a position deviation of an objective lens  27  at the time of access to a desired recording track on an optical disc  11  can be detected by finding the difference between the light quantity of the light received by the light-receiving sections A 1  and A 2  of the photodetector IC  44  and the light quantity of the light received by the light-receiving sections A 3  and A 4.

TECHNICAL FIELD

This invention relates to an integrated optical element used for anoptical pickup for recording and/or reproduction of signals byirradiating with a light beam a signal recording surface of an opticaltype disc (hereinafter referred to as optical disc) such as a mini disc(MD), a magneto-optical disc (MO), a compact disc (CD) or a CD-ROM, anoptical pickup using this integrated optical element, and an opticaldisc device having this optical pickup.

BACKGROUND ART

Conventionally, an optical pickup constituted, for example, as shown inFIG. 1 is proposed as an optical pickup for an optical disc. The opticalpickup 1 shown in FIG. 1 has a semiconductor laser element 2 as a lightsource, an optical member 3 made of a transparent material such as glassor plastics, an objective lens 4 for converging a light, and aphotodetector 5 for receiving a light and converting the light to anelectric signal.

Of these elements constituting the optical pickup 1, the light-emittingelement 2, the optical member 3 and the photodetector 5 are integratedas an integrated element, which is provided in the state of being fixedto a base, not shown. The objective lens 4 is provided on the base via abiaxial actuator, not shown. As the biaxial actuator is driven, theobjective lens 4 is minutely moved in biaxial directions, that is, inthe direction of radius of an optical disc D and in the direction towardand away from the optical disc D.

In this optical pickup, as the base is fed in the direction of radius ofthe optical disc D by the driving of a thread feed motor, not shown, adesired recording track on the optical disc can be accessed.

In this optical pickup 1, the optical member 3 has two surfaces inparallel to each other. These two surfaces are arranged to besubstantially perpendicular to the optical axis of the light beam fromthe semiconductor laser element 2. On a first surface (lower-center sidein FIG. 1) of the optical member 3 on the side of the semiconductorlaser element 2, a grating 3 a for diffracting a light beam directedtoward the optical disc D into the direction corresponding to thedirection along the recording track on the optical disc D and forsplitting the light beam into a plurality of beams including at least amain beam and two side beams is formed on the optical axis of the lightbeam from the semiconductor laser element 2.

Also, on a second surface (upper-center surface in FIG. 1) of theoptical member 3 on the side of the optical disc D, a hologram 3 b fordiffracting a return light beam reflected from the signal recordingsurface of the optical disc D and for guiding the return light beam tothe photodetector 5 is formed on the optical axis of the light beam fromthe semiconductor laser element 2.

The hologram 3 b functions as optical path branching means for branchingthe optical path of the return light beam directed toward thephotodetector 5 from the optical path of the light beam directed towardthe optical disc D, by diffracting the return light beam from theoptical disc D incident on the second surface of the optical member 3and thus directing the return light beam toward the photodetector 5.

The hologram 3 b has two hologram areas 3 b-1 and 3 b-2 for diffractingthe incident return light beam by different diffraction angles,respectively, as shown in FIG. 1. The boundary between these hologramareas 3 b-1 and 3 b-2 is formed on the second surface of the opticalmember 3 so as to be substantially coincident with the directioncorresponding to the direction of radius of the optical disc D. Thereturn light beam from the optical disc D becomes incident on thehologram 3 b with its center located on the boundary between the twohologram areas 3 b-1 and 3 b-2, and is bisected along the boundary. Thebisected portions are diffracted by different diffraction angles,respectively.

That is, the hologram 3 b also functions as return light beam splittingmeans for bisecting the incident return light beam along a splittingline in the direction corresponding to the direction of radius of theoptical disc D.

The photodetector 5 has a center light-receiving section 5 a forreceiving a return light of the main beam, from among the plurality ofbeams generated by the grating 3 a, and light-receiving sections e and ffor receiving return lights of the side beams, provided on both sides ofthe center light-receiving section 5 a, specifically, on both sides ofthe direction corresponding to the direction along the recording trackon the optical disc D, as shown in FIG. 1. The center light-receivingsection 5 a further has four light-receiving sections a, b, c and dwhich are divided by a division line d1 in the direction correspondingto the direction of radius of the optical disc D and a division line d2in the direction along the recording track on the optical disc D.

The division line d1 on the center light-receiving section 5 a extendsin the direction corresponding to the direction of radius of the opticaldisc D along the direction of diffraction of the hologram 3 b in orderto prevent generation of any deviation of the focusing error signallevel of a light spot in a focused state due to a change in theoscillation wavelength of the semiconductor laser element 2 or due to achange in the refractive index of the optical member 3 by a temperaturechange.

In this optical pickup 1, the light beam emitted from the semiconductorlaser element 2 becomes incident on the optical member 3 from its firstsurface, then is split into a plurality of beams by the grating 3 a, andthen passes through the optical member 3. The optical beam which haspassed through the optical member 3 is converged by the objective lens 4and is cast as a fine light spot onto the signal recording surface ofthe optical disc D. In this case, three light spots are formed on thesignal recording surface of the optical disc D by the main beam and twoside beams generated by the grating 3 a. In FIG. 1, only the main beamis shown.

The light beams cast on the signal recording surface of the optical discD reflected by the signal recording surface of the optical disc D so asto be return light beams. The return light beams pass again through theobjective lens 4 and become incident on the optical member 3 from itssecond surface. The return light beams incident on the optical member 3are diffracted by the hologram 3 b formed on the second surface of theoptical member 3. Specifically, the return light beams become incidenton the hologram 3 b with their centers located on the boundary betweenthe two hologram areas 3 b-1 and 3 b-2, and the portions incident on theindividual hologram areas 3 b-1 and 3 b-2 are diffracted by differentdiffraction angles, respectively. The return light beams diffracted bythe hologram areas 3 b-1 and 3 b-2 of the hologram 3 b pass through theoptical member 3 so as to be directed toward the photodetector.

Of the return light beams directed toward the photodetector 5, thereturn light beam which is a return light of the main beam and isdiffracted by the one hologram area 3 b-1 of the hologram 3 b becomesincident on the two light receiving sections a and b of the centerlight-receiving section 5 a of the photodetector 5. Of the return lightbeams directed toward the photodetector 5, the return light beam whichis a return light of the main beam and is diffracted by the otherhologram area 3 b-2 of the hologram 3 b becomes incident on theremaining two light receiving sections c and d of the centerlight-receiving section 5 a of the photodetector 5. Of the return lightbeams directed toward the photodetector 5, the return light beams of theside beams become incident on the light-receiving sections e and f ofthe photodetector 5.

The photodetector 5 converts the lights incident on the light-receivingsections a, b, c, d, e and f to electric signals, and supplies theresultant signals to a signal processing circuit, not shown. Thedetection signals from the photodetector 5 are amplified by headamplifiers in the signal processing circuit so as to be output signalsSa, Sb, Sc, Sd, Se and Sf, and predetermined arithmetic processing isperformed thereon by an arithmetic circuit. Thus, a reproduction signalRF0 is generated. Also, a focusing error signal FE0 is generated by aso-called Foucault method and a tracking error signal TR0 is generatedby a three-beam method.

The reproduction signal RF0 is calculated, for example, by thearithmetic circuit carrying out arithmetic processing with the followingequation (1).

RF 0=(Sa+Sb)+(Sc+Sd)  (1)

The focusing error signal FE0 is calculated, for example, by thearithmetic circuit carrying out arithmetic processing with the followingequation (2), (3) or (4).

FE 0=Sa−Sb  (2)

FE 0=Sc−Sd  (3)

FE 0=(Sa+Sd)−(Sc+Sb)  (4)

The tracking error signal TR0 is calculated, for example, by thearithmetic circuit carrying out arithmetic processing with the followingequation (5).

TR 0=Se−Sf  (5)

On the basis of the focusing error signal FE0 thus generated, theoptical pickup 1 carries out focusing servo for driving the biaxialactuator to minutely move the objective lens 4 in the direction towardand away from the optical disc D and thus controls the focal point ofthe light beam converged by the objective lens 4 so as to be constantlylocated on the signal recording surface of the optical disc D.

In addition, on the basis of the tracking error signal TR0 generated inthe above-described manner, the optical pickup 1 carries out trackingservo for driving the biaxial actuator to minutely move the objectivelens 4 in the direction of radius of the optical disc D and thus causesthe spot of the light beam converged by the objective lens 4 to followthe recording track on the optical disc D.

By thus carrying out focusing servo and tracking servo whilerecording/reproducing signals to/from the optical disc D, the opticalpickup 1 can carry out appropriate signal recording/reproduction even inthe case where the optical disc D is fluctuated or tilted.

Meanwhile, in the case where signal recording/reproduction is to becarried out with the optical disc D in the optical pickup 1, the basemust be first fed in the direction of radius of the optical disc toaccess a predetermined recording track, as described above, in the statewhere tracking servo is off.

The objective lens 4 is provided movably on the base via the biaxialactuator as described above. Therefore, when the base is fed in thedirection of radius of the optical disc D or when the feed operation isstopped, the position of the objective lens with respect to the base isdeviated in the direction of radius of the optical disc D (to the innerside or outer side of the optical disc D) from the normal positionbecause of inertia, as shown in FIG. 2.

The deviation of the objective lens 4 from the normal position iseliminated with the lapse of a predetermined time period. However,during this time period for recovering the position of the objectivelens 4, recording/reproduction of signals to/from the optical disccannot be carried out. This is one element that substantially disturbsthe high-speed access property of the optical pickup 1.

To realize the high-speed property of such optical pickup 1, theposition of the objective lens 4 may be instantaneously recovered bydetecting the deviation of the objective lens 4 from the normalposition, then feeding the detected deviation back to the biaxialactuator, and controlling the driving of the biaxial actuator toeliminate the deviation of the objective lens 4.

To detect the deviation of the objective lens 4 from the normalposition, there is considered, for example, a technique of attaching aposition sensor to the objective lens 4 so as to detect the actualposition of the objective lens 4 by the position sensor and thusdetecting the position deviation of the objective lens 4 from the actualposition of the objective lens 4 and the quantity of feeding of thebase.

However, this technique requires the position sensor for detecting theposition of the objective lens 4 to be separately provided in theoptical pickup 1, and therefore causes problems such as the increase inthe number of components, increase in size of the optical pickup 1 andrise in cost.

DISCLOSURE OF THE INVENTION

In view of the foregoing status of the art, it is an object of thepresent invention to provide an optical pickup which realizes thehigh-speed access property by appropriately and simply detecting theposition deviation of the objective lens as the light beam convergingmeans at the time of access without adding any new component, anintegrated optical element used therefor, and an optical disc devicehaving this optical pickup.

An integrated optical element according to the present invention, usedfor an optical pickup for carrying out recording and/or reproduction ofsignals by irradiating a signal recording surface of an optical discwith a light beam, includes: a light source for emitting the light beam;a photodetector having a light-receiving section for receiving a returnlight beam reflected by the signal recording surface of the opticaldisc; a package member for housing the light source and thephotodetector therein; an optical member arranged on the package memberfor transmitting the light beam emitted from the light source and fortransmitting the return light beam directed toward the photodetector;and optical path branching means integrally formed with the opticalmember for separating the optical path of the light beam emitted fromthe light source and the optical path of the return light beam directedtoward the photodetector.

In this integrated optical element, the optical path branching means hasat least two diffraction areas for diffracting the return light beamreflected by the signal recording surface of the optical disc, intodifferent directions, respectively, and the boundary between thediffraction areas is inclined at a predetermined angle with respect tothe direction corresponding to the direction of radius of the opticaldisc. At least one light-receiving section of the photodetector isdivided into a portion for receiving a return light beam diffracted byone diffraction area of the optical path branching means and a portionfor receiving a return light beam diffracted by the other diffractionarea.

In the integrated optical element according to the present invention,the light source is housed inside the package member and emits the lightbeam for irradiating the signal recording surface of the optical disc.The light beam emitted from the light source is transmitted through theoptical member provided on the package member. Then, the light beamtransmitted through the optical member is converged by light beamconverging means of the optical pickup and then cast onto the signalrecording surface of the optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

Since the optical path branching means has at least two diffractionareas for diffracting the return light beam in different directions, thereturn light beams incident on the respective diffraction areas of theoptical path branching means are diffracted into different directions bythe diffraction areas of the optical path branching means, thentransmitted through the optical member, and directed toward thephotodetector housed in the package member.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving the return light beam diffractedby one diffraction area of the optical path branching means and theportion for receiving the return light beam diffracted by the otherdiffraction area, the return light beams diffracted into differentdirections by the respective diffraction areas of the optical pathbranching means are received by the corresponding portions of thelight-receiving section of the photodetector.

Thus, on the basis of the detection signal from the light-receivingsection of the photodetector, a reproduction signal is generated and afocusing error signal is generated by a so-called Foucault method.

Meanwhile, in the case where the position of the light beam convergingmeans with respect to the integrated optical element is deviated in thedirection of radius of the optical disc from the normal position at thetime of access to a desired recording track on the optical disc, thespot of the return light beam incident on the optical member is deviatedin the direction corresponding to the direction of radius of the opticaldisc, on the optical path branching means.

In the integrated optical element according to the present invention,the optical path branching means has at least two diffraction areas fordiffracting the return light beam into different directions, and theboundary between these diffraction areas is inclined at a predeterminedangle with respect to the direction corresponding to the direction ofradius of the optical disc. Therefore, in the case where the position ofthe light beam converging means is deviated in the direction of radiusof the optical disc from the normal position, the return light beam issplit asymmetrically by the optical path branching means. The individualparts of the return light beam generated by asymmetrical splitting bythe optical path branching means are received by the correspondingportions of the light-receiving section of the photodetector. Therefore,the position deviation of the light beam converging means with respectto the integrated optical element can be detected on the basis of thedetection signal from the light-receiving section of the photodetector.

According to the integrated optical element of the present invention,since the position deviation of the light beam converging means withrespect to the integrated optical element at the time of access to adesired recording track on the optical disc can be detected in theabove-described manner, the optical pickup using this integrated opticalelement can realize the high-speed access property.

Also, in this integrated optical element, the optical path branchingmeans for separating the optical path of the return light beam directedtoward the photodetector from the optical path of the light beam emittedfrom the light source and for generating a focusing error signal by theFoucault method is used for detecting the position deviation of thelight beam converging means with respect to the integrated opticalelement, instead of additionally providing any means for detecting theposition deviation of the light beam converging means. Therefore, theoptical pickup using this integrated optical element can realize thehigh-speed access property by appropriately and simply detecting theposition deviation of the light beam converging means, without causingthe increase in the number of components, the increase in the size ofthe device itself and the rise in cost.

In the integrated optical element according to the present invention, itis desired that light beam splitting means for diffracting the lightbeam directed toward the optical disc and for splitting the light beaminto a plurality of beams including a main beam and two side beams isformed integrally with the optical member.

As the light beam splitting means for diffracting and splitting thelight beam directed toward the optical disc into a plurality of beams isprovided, as described above, the integrated optical element accordingto the present invention forms spots of the main beam and two side beamsonto the signal recording surface of the optical disc and detects thereturn light beams thereof. Thus, a tracking error signal can begenerated by a so-called three-beam method.

Moreover, as the light beam splitting means is formed integrally withthe optical member, as described above, the integrated optical elementaccording to the present invention can realize miniaturization of theintegrated optical element itself and miniaturization of the opticalpickup using this integrated optical element.

Also, in the integrated optical element according to the presentinvention, it is desired that at least one light-receiving section ofthe photodetector is divided by division lines substantially parallel tothe boundary of the diffraction areas of the optical path branchingmeans.

As at least one light-receiving section of the photodetector is dividedby the division lines substantially parallel to the boundary of thediffraction areas of the optical path branching means, as describedabove, the integrated optical element according to the present inventioncan effectively restrain generation of any deviation of the signal levelof the focusing error signal due to a position change of the returnlight spot and detect an appropriate focusing error signal even in thecase where the position of the return light spot of the light beam in afocused state on the photodetector is somewhat changed by a change inthe oscillation wavelength of the light source or by a change in therefractive index of the optical member due to a temperature change.

Another integrated optical element according to the present invention,used for an optical pickup for carrying out recording and/orreproduction of signals by irradiating a signal recording surface of anoptical disc with a light beam, includes: a light source for emittingthe light beam; a photodetector having a light-receiving section forreceiving a return light beam reflected by the signal recording surfaceof the optical disc; a package member for housing the light source andthe photodetector therein; an optical member arranged on the packagemember for transmitting the light beam emitted from the light source andfor transmitting the return light beam directed toward thephotodetector; optical path branching means integrally formed with theoptical member for separating the optical path of the light beam emittedfrom the light source and the optical path of the return light beamreflected by the signal recording surface of the optical disc; andreturn light beam splitting means integrally formed with the opticalmember for splitting the return light beam passed through the opticalpath branching means into at least two beams.

In this integrated optical element, the return light beam splittingmeans has at least two surfaces having different normal vectors, and theboundary between these surfaces is inclined at a predetermined anglewith respect to a direction corresponding to the direction of radius ofthe optical disc. At least one light-receiving section of thephotodetector is divided into a portion for receiving one return lightbeam generated by the return light beam splitting means and a portionfor receiving the other return light beam.

In this another integrated optical element according to the presentinvention, the light source is housed inside the package member andemits the light beam for irradiating the signal recording surface of theoptical disc. The light beam emitted from the light source istransmitted through the optical member provided on the package member.Then, the light beam transmitted through the optical member is convergedby light beam converging means of the optical pickup and then cast ontothe signal recording surface of the optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

The return light beam having its optical path branched by the opticalpath branching means is then split into at least two beams by the returnlight beam splitting means integrally formed with the optical member.

Since the return light beam splitting means has at least two surfaceshaving different normal vectors, the return light beam incident on thereturn light beam splitting means is split along the boundary betweenthe surfaces. The beams generated by splitting proceed in differentdirections in accordance with the normal vectors of the surfaces onwhich they are incident, and are directed toward the photodetectorhoused in the package member.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving one return light beam generatedby the return light beam splitting means and the portion for receivingthe other return light beam, the return light beams generated by thereturn light beam splitting means are received by the correspondingportions of the light-receiving section of the photodetector.

Thus, on the basis of the detection signal from the light-receivingsection of the photodetector, a reproduction signal is generated and afocusing error signal is generated by a so-called Foucault method.

Meanwhile, in the case where the position of the light beam convergingmeans with respect to the integrated optical element is deviated in thedirection of radius of the optical disc from the normal position at thetime of access to a desired recording track on the optical disc, thespot of the return light beam incident on the optical member is deviatedin the direction corresponding to the direction of radius of the opticaldisc, on the optical path branching means.

In this another integrated optical element according to the presentinvention, the return light beam splitting means has at least twosurfaces of different normal vectors and the boundary between thesesurfaces is inclined at a predetermined angle with respect to thedirection corresponding to the direction of radius of the optical disc.Therefore, in the case where the position of the light beam convergingmeans is deviated in the direction of radius of the optical disc fromthe normal position, the return light beam is split asymmetrically bythe return light beam splitting means. The individual parts of thereturn light beam generated by asymmetrical splitting by the returnlight beam splitting means are received by the corresponding portions ofthe light-receiving section of the photodetector. Therefore, theposition deviation of the light beam converging means with respect tothe integrated optical element can be detected on the basis of thedetection signal from the light-receiving section of the photodetector.

According to this another integrated optical element of the presentinvention, since the position deviation of the light beam convergingmeans with respect to the integrated optical element at the time ofaccess to a desired recording track on the optical disc can be detectedin the above-described manner, the optical pickup using this integratedoptical element can realize the high-speed access property.

Also, in this another integrated optical element, the return light beamsplitting means for generating a focusing error signal by the Foucaultmethod is used for detecting the position deviation of the light beamconverging means with respect to the integrated optical element, insteadof additionally providing any means for detecting the position deviationof the light beam converging means. Therefore, the optical pickup usingthis integrated optical element can realize the high-speed accessproperty by appropriately and simply detecting the position deviation ofthe light beam converging means, without causing the increase in thenumber of components, the increase in the size of the device itself andthe rise in cost.

In this another integrated optical element according to the presentinvention, it is desired that light beam splitting means for diffractingthe light beam directed toward the optical disc and for splitting thelight beam into a plurality of beams including a main beam and two sidebeams is formed integrally with the optical member.

As the light beam splitting means for diffracting and splitting thelight beam directed toward the optical disc into a plurality of beams isprovided, as described above, this another integrated optical elementaccording to the present invention forms spots of the main beam and twoside beams onto the signal recording surface of the optical disc anddetects the return light beams thereof. Thus, a tracking error signalcan be generated by a so-called three-beam method.

Moreover, as the light beam splitting means is formed integrally withthe optical member, as described above, this another integrated opticalelement according to the present invention can realize miniaturizationof the integrated optical element itself and miniaturization of theoptical pickup using this integrated optical element.

Also, in this another integrated optical element according to thepresent invention, it is desired that at least one light-receivingsection of the photodetector is divided by division lines substantiallyparallel to the boundary of the return light beam splitting means.

As at least one light-receiving section of the photodetector is dividedby the division lines substantially parallel to the boundary of thediffraction areas of the optical path branching means, as describedabove, the integrated optical element according to the present inventioncan effectively restrain generation of any deviation of the signal levelof the focusing error signal due to a position change of the returnlight spot and detect an appropriate focusing error signal even in thecase where the position of the return light spot of the light beam in afocused state on the photodetector is somewhat changed by a change inthe oscillation wavelength of the light source or by a change in therefractive index of the optical member due to a temperature change.

An optical pickup according to the present invention for carrying outrecording and/or reproduction of signals by irradiating a signalrecording surface of an optical disc with a light beam includes: a lightsource for emitting the light beam; light beam converging means forconverging the light beam emitted from the light source and forirradiating the signal recording surface of the optical disc with theconverged light beam; a photodetector having a light-receiving sectionfor receiving a return light beam reflected by the signal recordingsurface of the optical disc; an optical member arranged between thelight source and photodetector on one side and the light beam convergingmeans on the other side, for transmitting the light beam emitted fromthe light source and for transmitting the return light beam directedtoward the photodetector; optical path branching means integrally formedwith the optical member for separating the optical path of the lightbeam emitted from the light source and the optical path of the returnlight beam directed toward the photodetector; and a biaxial actuator formoving the light beam converging means into biaxial directions, that is,the direction of radius of the optical disc and the direction toward andaway from the optical disc.

In this optical pickup, the optical path branching means has at leasttwo diffraction areas for diffracting the return light beam reflected bythe signal recording surface of the optical disc, into differentdirections, respectively, and the boundary between the diffraction areasis inclined at a predetermined angle with respect to the directioncorresponding to the direction of radius of the optical disc. At leastone light-receiving section of the photodetector is divided into aportion for receiving a return light beam diffracted by one diffractionarea of the optical path branching means and a portion for receiving areturn light beam diffracted by the other diffraction area.

In the optical pickup according to the present invention, the light beamemitted from the light source is transmitted through the optical memberand directed toward the light beam converging means. The light beam isthen converged by light beam converging means and then cast onto thesignal recording surface of the optical disc.

In the case where the spot of the light beam cast on the signalrecording surface of the optical disc is deviated from a predeterminedrecording track on the signal recording surface of the optical disc, orin the case where no focal point is formed on the signal recordingsurface of the optical disc, the biaxial actuator is driven on the basisof a tracking error signal or a focusing error signal, and the lightbeam converging means is moved by the biaxial actuator into thedirection of radius of the optical disc or into the direction toward andaway from the optical disc. Thus, the spot of the light beam convergedby the light beam converging means and cast onto the signal recordingsurface of the optical disc constantly follows a predetermined recordingtrack on the signal recording surface of the optical disc and forms afocal point on the signal recording surface of the optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

Since the optical path branching means has at least two diffractionareas for diffracting the return light beam in different directions, thereturn light beams incident on the respective diffraction areas of theoptical path branching means are diffracted into different directions bythe diffraction areas of the optical path branching means, thentransmitted through the optical member, and directed toward thephotodetector.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving the return light beam diffractedby one diffraction area of the optical path branching means and theportion for receiving the return light beam diffracted by the otherdiffraction area, the return light beams diffracted into differentdirections by the respective diffraction areas of the optical pathbranching means are received by the corresponding portions of thelight-receiving section of the photodetector.

Thus, on the basis of the detection signal from the light-receivingsection of the photodetector, a reproduction signal is generated and afocusing error signal is generated by a so-called Foucault method.

Meanwhile, in the case where the position of the light beam convergingmeans with respect to the optical member is deviated in the direction ofradius of the optical disc from the normal position at the time ofaccess to a desired recording track on the optical disc, the spot of thereturn light beam incident on the optical member is deviated in thedirection corresponding to the direction of radius of the optical disc,on the optical path branching means.

In the optical pickup according to the present invention, the opticalpath branching means has at least two diffraction areas for diffractingthe return light beam into different directions, and the boundarybetween these diffraction areas is inclined at a predetermined anglewith respect to the direction corresponding to the direction of radiusof the optical disc. Therefore, in the case where the position of thelight beam converging means is deviated in the direction of radius ofthe optical disc from the normal position, the return light beam issplit asymmetrically by the optical path branching means. The individualparts of the return light beam generated by asymmetrical splitting bythe optical path branching means are received by the correspondingportions of the light-receiving section of the photodetector. Therefore,the position deviation of the light beam converging means with respectto the optical member can be detected on the basis of the detectionsignal from the light-receiving section of the photodetector.

According to the optical pickup of the present invention, since theposition deviation of the light beam converging means with respect tothe optical member at the time of access to a desired recording track onthe optical disc can be detected in the above-described manner, theposition of the light beam converging means can be instantaneouslyrecovered to secure the accuracy and stability of the access operationand to realize the high-speed access property.

Also, in this optical pickup according to the present invention, theoptical path branching means for separating the optical path of thereturn light beam directed toward the photodetector from the opticalpath of the light beam emitted from the light source and for generatinga focusing error signal by the Foucault method is used for detecting theposition deviation of the light beam converging means with respect tothe optical member, instead of additionally providing any means fordetecting the position deviation of the light beam converging means.Therefore, the optical pickup can realize the high-speed access propertyby appropriately and simply detecting the position deviation of thelight beam converging means, without causing the increase in the numberof components, the increase in the size of the device itself and therise in cost.

Another optical pickup according to the present invention for carryingout recording and/or reproduction of signals by irradiating a signalrecording surface of an optical disc with a light beam includes: a lightsource for emitting the light beam; light beam converging means forconverging the light beam emitted from the light source and forirradiating the signal recording surface of the optical disc with theconverged light beam; a photodetector having a light-receiving sectionfor receiving a return light beam reflected by the signal recordingsurface of the optical disc; an optical member arranged between thelight source and photodetector on one side and the light beam convergingmeans on the other side, for transmitting the light beam emitted fromthe light source and for transmitting the return light beam directedtoward the photodetector; optical path branching means integrally formedwith the optical member for separating the optical path of the lightbeam emitted from the light source and the optical path of the returnlight beam reflected by the signal recording surface of the opticaldisc; return light beam splitting means integrally formed with theoptical member for splitting the return light beam passed through theoptical path branching means into at least two beams; and a biaxialactuator for moving the light beam converging means into biaxialdirections, that is, the direction of radius of the optical disc and thedirection toward and away from the optical disc.

In this optical pickup, the return light beam splitting means has atleast two surfaces having different normal vectors, and the boundarybetween these surfaces is inclined at a predetermined angle with respectto a direction corresponding to the direction of radius of the opticaldisc. At least one light-receiving section of the photodetector isdivided into a portion for receiving one return light beam generated bythe return light beam splitting means and a portion for receiving theother return light beam.

In this another optical member according to the present invention, thelight beam emitted from the light source is transmitted through theoptical member and directed toward the light beam converging means.Then, the light beam is converged by light beam converging means andthen cast onto the signal recording surface of the optical disc.

In the case where the spot of the light beam cast on the signalrecording surface of the optical disc is deviated from a predeterminedrecording track on the signal recording surface of the optical disc, orin the case where no focal point is formed on the signal recordingsurface of the optical disc, the biaxial actuator is driven on the basisof a tracking error signal or a focusing error signal, and the lightbeam converging means is moved by the biaxial actuator into thedirection of radius of the optical disc or into the direction toward andaway from the optical disc. Thus, the spot of the light beam convergedby the light beam converging means and cast onto the signal recordingsurface of the optical disc constantly follows a predetermined recordingtrack on the signal recording surface of the optical disc and forms afocal point on the signal recording surface of the optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

The return light beam having its optical path branched by the opticalpath branching means is then split into at least two beams by the returnlight beam splitting means integrally formed with the optical member.

Since the return light beam splitting means has at least two surfaceshaving different normal vectors, the return light beam incident on thereturn light beam splitting means is split along the boundary betweenthe surfaces. The beams generated by splitting proceed in differentdirections in accordance with the normal vectors of the surfaces onwhich they are incident, and are directed toward the photodetector.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving one return light beam generatedby the return light beam splitting means and the portion for receivingthe other return light beam, the return light beams generated by thereturn light beam splitting means are received by the correspondingportions of the light-receiving section of the photodetector.

Thus, on the basis of the detection signal from the light-receivingsection of the photodetector, a reproduction signal is generated and afocusing error signal is generated by a so-called Foucault method.

Meanwhile, in the case where the position of the light beam convergingmeans with respect to the optical member is deviated in the direction ofradius of the optical disc from the normal position at the time ofaccess to a desired recording track on the optical disc, the spot of thereturn light beam incident on the optical member is deviated in thedirection corresponding to the direction of radius of the optical disc,on the optical path branching means.

In this another optical pickup according to the present invention, thereturn light beam splitting means has at least two surfaces of differentnormal vectors and the boundary between these surfaces is inclined at apredetermined angle with respect to the direction corresponding to thedirection of radius of the optical disc. Therefore, in the case wherethe position of the light beam converging means is deviated in thedirection of radius of the optical disc from the normal position, thereturn light beam is split asymmetrically by the return light beamsplitting means. The individual parts of the return light beam generatedby asymmetrical splitting by the return light beam splitting means arereceived by the corresponding portions of the light-receiving section ofthe photodetector. Therefore, the position deviation of the light beamconverging means with respect to the optical member can be detected onthe basis of the detection signal from the light-receiving section ofthe photodetector.

According to this another optical pickup of the present invention, sincethe position deviation of the light beam converging means with respectto the optical member at the time of access to a desired recording trackon the optical disc can be detected in the above-described manner, theposition of the light beam converging means can be instantaneouslyrecovered to secure the accuracy and stability of the access operationand to realize the high-speed access property.

Also, in this another optical pickup according to the present invention,the return light beam splitting means for generating a focusing errorsignal by the Foucault method is used for detecting the positiondeviation of the light beam converging means with respect to the opticalmember, instead of additionally providing any means for detecting theposition deviation of the light beam converging means. Therefore, theoptical pickup can realize the high-speed access property byappropriately and simply detecting the position deviation of the lightbeam converging means, without causing the increase in the number ofcomponents, the increase in the size of the device itself and the risein cost.

An optical disc device according to the present invention includes: discrotating means for rotating an optical disc; an optical pickup forcarrying out recording and/or reproduction of signals by irradiatingwith a light beam a signal recording surface of the optical disc rotatedby the disc rotating means; a signal processing circuit for processing adetection signal from the optical pickup; and an access mechanism formoving the optical pickup in the direction of radius of the opticaldisc.

In this optical disc device, the optical pickup includes a light sourcefor emitting the light beam, light beam converging means for convergingthe light beam emitted from the light source and for irradiating thesignal recording surface of the optical disc with the converged lightbeam, a photodetector having a light-receiving section for receiving areturn light beam reflected by the signal recording surface of theoptical disc, an optical member arranged between the light source andphotodetector on one side and the light beam converging means on theother side, for transmitting the light beam emitted from the lightsource and for transmitting the return light beam directed toward thephotodetector, optical path branching means integrally formed with theoptical member for separating the optical path of the light beam emittedfrom the light source and the optical path of the return light beamdirected toward the photodetector, and a biaxial actuator for moving thelight beam converging means into biaxial directions, that is, thedirection of radius of the optical disc and the direction toward andaway from the optical disc. The optical path branching means has atleast two diffraction areas for diffracting the return light beamreflected by the signal recording surface of the optical disc, intodifferent directions, respectively, and the boundary between thediffraction areas is inclined at a predetermined angle with respect tothe direction corresponding to the direction of radius of the opticaldisc. At least one light-receiving section of the photodetector isdivided into a portion for receiving a return light beam diffracted byone diffraction area of the optical path branching means and a portionfor receiving a return light beam diffracted by the other diffractionarea.

In the optical disc device according to the present invention, theoptical disc as a recording medium is rotated by the disc rotatingmeans. Then, when signals are to be recorded to and/or reproduced fromthe optical disc rotated by the disc rotating means, a light beam isfirst emitted from the light source of the optical pickup to the opticaldisc rotated by the disc rotating means. The optical pickup is moved inthe direction of radius of the optical disc by the access mechanism soas to access a desired recording track.

The light beam emitted from the light source is transmitted through theoptical member and directed toward the light beam converging means. Thelight beam is then converged by light beam converging means and thencast onto the signal recording surface of the optical disc.

In the case where the spot of the light beam cast on the signalrecording surface of the optical disc is deviated from a predeterminedrecording track on the signal recording surface of the optical disc, orin the case where no focal point is formed on the signal recordingsurface of the optical disc, the biaxial actuator is driven on the basisof a tracking error signal or a focusing error signal from the signalprocessing circuit, and the light beam converging means is moved by thebiaxial actuator into the direction of radius of the optical disc orinto the direction toward and away from the optical disc. Thus, the spotof the light beam converged by the light beam converging means and castonto the signal recording surface of the optical disc constantly followsa predetermined recording track on the signal recording surface of theoptical disc and forms a focal point on the signal recording surface ofthe optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

Since the optical path branching means has at least two diffractionareas for diffracting the return light beam in different directions, thereturn light beams incident on the respective diffraction areas of theoptical path branching means are diffracted into different directions bythe diffraction areas of the optical path branching means, thentransmitted through the optical member, and directed toward thephotodetector.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving the return light beam diffractedby one diffraction area of the optical path branching means and theportion for receiving the return light beam diffracted by the otherdiffraction area, the return light beams diffracted into differentdirections by the respective diffraction areas of the optical pathbranching means are received by the corresponding portions of thelight-receiving section of the photodetector.

The return light beams received by the respective portions of thelight-receiving section of the photodetector are photoelectricallyconverted by the photodetector and supplied as detection signals to thesignal processing circuit. In the signal processing circuit,predetermined arithmetic processing is carried out on the basis of thedetection signal, thereby generating a reproduction signal andgenerating a focusing error signal by a so-called Foucault method.

Meanwhile, in the case where signals are to be recorded to and/orreproduced from the optical disc in this optical disc device, first theoptical pickup is moved in the direction of radius by the accessmechanism so as to access a desired recording track, as described above.In this case, the position of the light beam converging means of theoptical pickup with respect to the optical member may be deviated intothe direction of radius of the optical disc from the normal positionbecause of inertia. When the position of the light beam converging meanswith respect to the optical member is deviated in the direction ofradius of the optical disc from the normal position, the spot of thereturn light beam incident on the optical member is deviated in thedirection corresponding to the direction of radius of the optical disc,on the optical path branching means.

In the optical disc device according to the present invention, theoptical path branching means of the optical pickup has at least twodiffraction areas for diffracting the return light beam into differentdirections, and the boundary between these diffraction areas is inclinedat a predetermined angle with respect to the direction corresponding tothe direction of radius of the optical disc. Therefore, in the casewhere the position of the light beam converging means is deviated in thedirection of radius of the optical disc from the normal position, thereturn light beam is split asymmetrically by the optical path branchingmeans. The individual parts of the return light beam generated byasymmetrical splitting by the optical path branching means are receivedby the corresponding portions of the light-receiving section of thephotodetector. Therefore, the position deviation of the light beamconverging means with respect to the optical member can be detected bycarrying out predetermined arithmetic processing in the signalprocessing circuit on the basis of the detection signal from thelight-receiving section of the photodetector.

According to the optical disc device of the present invention, since theposition deviation of the light beam converging means with respect tothe optical member at the time when causing the optical pickup to accessa desired recording track on the optical disc can be detected in theabove-described manner, the position of the light beam converging meanscan be instantaneously recovered to secure the accuracy and stability ofthe access operation and to realize the high-speed access property.

Also, in this optical disc device according to the present invention,the optical path branching means of the optical pickup for separatingthe optical path of the return light beam directed toward thephotodetector from the optical path of the light beam emitted from thelight source and for generating a focusing error signal by the Foucaultmethod is used for detecting the position deviation of the light beamconverging means with respect to the optical member, instead ofadditionally providing any means for detecting the position deviation ofthe light beam converging means. Therefore, the optical disc device canrealize the high-speed access property by appropriately and simplydetecting the position deviation of the light beam converging means,without causing the increase in the number of components, the increasein the size of the device itself and the rise in cost.

Another optical disc device according to the present invention includes:disc rotating means for rotating an optical disc; an optical pickup forcarrying out recording and/or reproduction of signals by irradiatingwith a light beam a signal recording surface of the optical disc rotatedby the disc rotating means; a signal processing circuit for processing adetection signal from the optical pickup; and an access mechanism formoving the optical pickup in the direction of radius of the opticaldisc.

In this optical disc device, the optical pickup includes a light sourcefor emitting the light beam, light beam converging means for convergingthe light beam emitted from the light source and for irradiating thesignal recording surface of the optical disc with the converged lightbeam, a photodetector having a light-receiving section for receiving areturn light beam reflected by the signal recording surface of theoptical disc, an optical member arranged between the light source andphotodetector on one side and the light beam converging means on theother side, for transmitting the light beam emitted from the lightsource and for transmitting the return light beam directed toward thephotodetector, optical path branching means integrally formed with theoptical member for separating the optical path of the light beam emittedfrom the light source and the optical path of the return light beamreflected by the signal recording surface of the optical disc, returnlight beam splitting means integrally formed with the optical member forsplitting the return light beam passed through the optical pathbranching means into at least two beams, and a biaxial actuator formoving the light beam converging means into biaxial directions, that is,the direction of radius of the optical disc and the direction toward andaway from the optical disc. The return light beam splitting means has atleast two surfaces having different normal vectors, and the boundarybetween these surfaces is inclined at a predetermined angle with respectto a direction corresponding to the direction of radius of the opticaldisc. At least one light-receiving section of the photodetector isdivided into a portion for receiving one return light beam generated bythe return light beam splitting means and a portion for receiving theother return light beam.

In this another optical disc device according to the present invention,the optical disc as a recording medium is rotated by the disc rotatingmeans. Then, when signals are to be recorded to and/or reproduced fromthe optical disc rotated by the disc rotating means, a light beam isfirst emitted from the light source of the optical pickup to the opticaldisc rotated by the disc rotating means. The optical pickup is moved inthe direction of radius of the optical disc by the access mechanism soas to access a desired recording track.

The light beam emitted from the light source is transmitted through theoptical member and directed toward the light beam converging means. Thelight beam is then converged by light beam converging means and thencast onto the signal recording surface of the optical disc.

In the case where the spot of the light beam cast on the signalrecording surface of the optical disc is deviated from a predeterminedrecording track on the signal recording surface of the optical disc, orin the case where no focal point is formed on the signal recordingsurface of the optical disc, the biaxial actuator is driven on the basisof a tracking error signal or a focusing error signal from the signalprocessing circuit, and the light beam converging means is moved by thebiaxial actuator into the direction of radius of the optical disc orinto the direction toward and away from the optical disc. Thus, the spotof the light beam converged by the light beam converging means and castonto the signal recording surface of the optical disc constantly followsa predetermined recording track on the signal recording surface of theoptical disc and forms a focal point on the signal recording surface ofthe optical disc.

The light beam cast on the signal recording surface of the optical discis reflected by the signal recording surface of the optical disc andthus becomes a return light beam including a signal component. Thereturn light beam passes again through the light beam converging meansand then becomes incident on the optical member.

The return light beam incident on the optical member has its opticalpath separated from the optical path of the light beam emitted from thelight source, by the optical path branching means formed integrally withthe optical member. Specifically, the optical path branching meansincludes, for example, a hologram formed on the surface of the opticalmember. As the return light beam is diffracted by the hologram into thedirection toward the photodetector, the optical path of the return lightbeam is separated from the optical path of the light beam emitted fromthe light source.

The return light beam having its optical path branched by the opticalpath branching means is then split into at least two beams by the returnlight beam splitting means integrally formed with the optical member.

Since the return light beam splitting means has at least two surfaceshaving different normal vectors, the return light beam incident on thereturn light beam splitting means is split along the boundary betweenthe surfaces. The beams generated by splitting proceed in differentdirections in accordance with the normal vectors of the surfaces onwhich they are incident, and are directed toward the photodetector.

Since at least one light receiving section of the photodetector isdivided into the portion for receiving one return light beam generatedby the return light beam splitting means and the portion for receivingthe other return light beam, the return light beams generated by thereturn light beam splitting means are received by the correspondingportions of the light-receiving section of the photodetector.

The return light beams received by the respective portions of thelight-receiving section of the photodetector are photoelectricallyconverted by the photodetector and supplied as detection signals to thesignal processing circuit. In the signal processing circuit,predetermined arithmetic processing is carried out on the basis of thedetection signal, thereby generating a reproduction signal andgenerating a focusing error signal by a so-called Foucault method.

Meanwhile, in the case where signals are to be recorded to and/orreproduced from the optical disc in this optical disc device, first theoptical pickup is moved in the direction of radius by the accessmechanism so as to access a desired recording track, as described above.In this case, the position of the light beam converging means of theoptical pickup with respect to the optical member may be deviated intothe direction of radius of the optical disc from the normal positionbecause of inertia. When the position of the light beam converging meanswith respect to the optical member is deviated in the direction ofradius of the optical disc from the normal position, the spot of thereturn light beam incident on the optical member is deviated in thedirection corresponding to the direction of radius of the optical disc,on the optical path branching means.

In this another optical disc device according to the present invention,the return light beam splitting means of the optical pickup has at leasttwo surfaces of different normal vectors and the boundary between thesesurfaces is inclined at a predetermined angle with respect to thedirection corresponding to the direction of radius of the optical disc.Therefore, in the case where the position of the light beam convergingmeans is deviated in the direction of radius of the optical disc fromthe normal position, the return light beam is split asymmetrically bythe return light beam splitting means. The individual parts of thereturn light beam generated by asymmetrical splitting by the returnlight beam splitting means are received by the corresponding portions ofthe light-receiving section of the photodetector. Therefore, theposition deviation of the light beam converging means with respect tothe optical member can be detected by carrying out predeterminedarithmetic processing in the signal processing circuit on the basis ofthe detection signal from the light-receiving section of thephotodetector.

According to this another optical disc device of the present invention,since the position deviation of the light beam converging means withrespect to the optical member at the time when causing the opticalpickup to access a desired recording track on the optical disc can bedetected in the above-described manner, the position of the light beamconverging means can be instantaneously recovered to secure the accuracyand stability of the access operation and to realize the high-speedaccess property.

Also, in this another optical disc device according to the presentinvention, the return light beam splitting means of the optical pickupfor generating a focusing error signal by the Foucault method is usedfor detecting the position deviation of the light beam converging meanswith respect to the optical member, instead of additionally providingany means for detecting the position deviation of the light beamconverging means. Therefore, the optical disc device can realize thehigh-speed access property by appropriately and simply detecting theposition deviation of the light beam converging means, without causingthe increase in the number of components, the increase in the size ofthe device itself and the rise in cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a conventionaloptical pickup.

FIG. 2 is a view for explaining the deviation of an objective lens inthe direction of radius of an optical disc at the time of access by theoptical pickup.

FIG. 3 is a block diagram showing an exemplary structure of an opticaldisc device to which the present invention is applied.

FIG. 4 is a perspective view schematically showing an optical pickup towhich the present invention is applied.

FIG. 5 is a side view schematically showing the state inside the opticalpickup for explaining the positional relation of an integrated opticalelement to which the present invention is applied in the optical pickup.

FIG. 6 is a side view schematically showing the state inside theintegrated optical element to which the present invention is applied.

FIG. 7 is a perspective view schematically showing the positionalrelation of individual members constituting the optical pickup to whichthe present invention is applied.

FIGS. 8A to 8C show the relation between the position of an objectivelens in the optical pickup to which the present invention is applied andthe position of a return light spot formed on a hologram. FIG. 8A showsthe state where the objective lens is located at a normal position.FIGS. 8B and 8C show the state where the position of the objective lensis deviated.

FIG. 9 is a side view schematically showing the state inside anotherintegrated optical element to which the present invention is applied.

FIG. 10 is a perspective view schematically showing the positionalrelation of individual members constituting another optical pickup towhich the present invention is applied.

FIG. 11 is an enlarged perspective view showing essential portions of anoptical member provided in another optical pickup to which the presentinvention is applied.

FIGS. 12A to 12C show the relation between the position of an objectivelens in another optical pickup to which the present invention is appliedand the position of a return light spot formed on a Foucault prism. FIG.12A shows the state where the objective lens is located at a normalposition. FIGS. 12B and 12C show the state where the position of theobjective lens is deviated.

FIG. 13 is a side view schematically showing the state inside stillanother integrated optical element to which the present invention isapplied.

FIG. 14 is a perspective view schematically showing the positionalrelation of individual members constituting still another optical pickupto which the present invention is applied.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described indetail with reference to the drawings.

First Embodiment

FIG. 3 shows the overall structure of an exemplary optical disc deviceto which the present invention is applied. This optical disc device 10has a spindle motor 12 for rotationally driving an optical disc 11 as arecording medium, an optical pickup 20 for irradiating with a light beamthe signal recording surface of the optical disc 11 rotationally drivenby the spindle motor 12 and for receiving a return light beam reflectedby the signal recording surface of the optical disc 11 so as to readrecord signals recorded on the signal recording surface of the opticaldisc 11, and a control section 13 for controlling the spindle motor 12and the optical pickup 20, as shown in FIG. 3.

The control section 13 has an optical disc controller 14, a signalprocessing circuit 15, an interface 17, ahead access control section 18,and a servo control section 19.

The optical disc controller 14 controls the driving of the spindle motor12 with a predetermined number of rotations, and controls the operationof each part inside the control section 13.

The signal processing circuit 15 generates a reproduction signal basedon a detection signal from the optical pickup 20, and transmits thegenerated reproduction signal to an external computer or the like viathe interface 17. This allows the external computer or the like toreceive the signal recorded on the optical disc 11 as the reproductionsignal. The signal processing circuit 15 also generates control signalssuch as a tracking error signal, a focusing error signal and a signalindicating the position information of an objective lens on the basis ofthe detection signal from the optical pickup 20, and supplies thegenerated signals to the optical disc controller 14.

The head access control section 18 moves the optical pickup 20 at a highspeed into the direction of radius of the optical disc 11 under thecontrol of the optical disc controller 14 so as to cause the opticalpickup 20 to access a predetermined recording track on the signalrecording surface of the optical disc 11, for example, by track jump orthe like.

The servo control section 19 drives a biaxial actuator of the opticalpickup 20 on the basis of the tracking error signal or focusing errorsignal under the control of the optical disc controller 14 so as tominutely move the objective lens held by the biaxial actuator intobiaxial directions, that is, the direction of radius of the optical disc11 (i.e., tracking direction) and the direction toward and away from thesignal recording surface of the optical disc 11 (i.e., focusingdirection), thus carrying out focusing servo and tracking servo.

The servo control section 19 also drives the biaxial actuator of theoptical pickup 20 on the basis of the signal indicating the positioninformation of the objective lens so as to minutely move the objectivelens held by the biaxial actuator into the direction of radius of theoptical disc 11, thus carrying out so-called midpoint servo forcorrecting the position deviation of the objective lens in the directionof radius of the optical disc at the time when the optical pickup isoperated for access.

The optical pickup 20 has a base 21 which is connected to a thread motorof the head access control section 18 via a thread feed shaft 22, asshown in FIG. 4.

The base 21 is made of a metal material such as aluminum or the likeshaped in a plate-like form, and a through-hole 23 penetrating in thedirection of width is provided at one end thereof. As the thread feedshaft 22 is inserted into the through-hole 23, the base 21 is connectedto the thread motor of the head access control section 18 via the threadfeed shaft 22 and is made movable in the direction of radius of theoptical disc 11 indicated by an arrow X in FIG. 4 by the driving of thethread motor.

As the base 21 is moved in the direction of radius of the optical disc11 by the driving of the thread motor of the head access control section18, the optical pickup 20 can access a predetermined recording track onthe signal recording surface of the optical disc 11.

At the other end of the base 21, an engagement piece 25 to be engagedwith a guide shaft 24 of the optical disc device 10 is provided. As theengagement piece 25 is engaged with the guide shaft 24, the base 21 canmaintain its attitude and be stably moved by the driving of the threadmotor.

On the base 21, a biaxial actuator housing section 26 is provided whichopens in a main surface section facing the optical disc 11. Inside thebiaxial actuator housing section 26, a biaxial actuator 28 is housedwhich is adapted for minutely moving an objective lens 27 into thedirection of radius of the optical disc 11 indicated by an arrow X inFIG. 4 and into the direction toward and away from the signal recordingsurface of the optical disc 11 indicated by an arrow Z in FIG. 4.

The biaxial actuator 28 has a fixed portion 29 fixed to the base 21, alens holding portion 31 movably supported to the fixed portion 29 via asuspension 30, and a driving portion 32 having a coil and a magnet forminutely moving the lens holding portion 31 in biaxial directions by anelectromagnetic force. The objective lens 27 is held by the lens holdingportion 31 of the biaxial actuator 28.

In this biaxial actuator 28, when a current based on a tracking errorsignal and a focusing error signal is supplied to the coil of thedriving portion 32 from the servo control section 19, the drivingportion 32 generates an electromagnetic force to minutely move the lensholding portion 31 in the biaxial directions in accordance with thevalue of the current supplied thereto. Thus, the objective lens 27 heldby the lens holding portion 31 is minutely moved in the biaxialdirections, that is, in the direction of radius of the optical disc 11and in the direction toward and away from the optical disc 11, inaccordance with the tracking error signal and the focusing error signal.

At the other end of the base 21, an integrated optical element 40 ismounted which is formed as a one chip including a semiconductor laserelement as a light source and a photodetector integrated therein.

In this optical pickup 20, the integrated optical element 40 is mountedon the base 21 so that a light beam is emitted in a directionsubstantially parallel to the main surface portion of the base 21, thatis, substantially parallel to the signal recording surface of theoptical disc 11, as shown in FIG. 5. In this optical pickup 20, a lightbeam emitted from the integrated optical element 40 is reflected by areflection surface 32 a of a rise mirror 32, thereby bending the opticalpath by approximately 90 degrees to leading the optical path to theobjective lens 27.

As the light beam emitted from the integrated optical element 40 iscaused to proceed substantially in parallel to the signal recordingsurface of the optical disc 11, the optical pickup 20 can be reduced inthickness while an optical path length necessary for the light beam issecured.

The integrated optical element 40 has a semiconductor laser element 41as a light source, a prism 42 having a function to bend the optical pathof the light beam emitted from the semiconductor laser element 41, anoptical member 43 made of a transparent material for transmitting thelaser beam with its optical path bent by the prism 42 and fortransmitting a return light beam reflected by the signal recordingsurface of the optical disc 11, and a photodetector IC 44 as aphotodetector for receiving the return light beam, as shown in FIG. 6.

The semiconductor laser element 41, the prism 42 and the photodetectorIC 44 are provided on a board 46 arranged inside a package member 45. Onone main surface portion of the package member 45, an aperture 45 a isprovided. On the main surface portion of the package member 45 where theaperture 45 a is provided, the optical member 43 is joined by anadhesive or the like so as to close the aperture 45 a. That is, theintegrated optical element 40 is constituted as one element includingthe individual members integrated therein. This integrated opticalelement 40 is mounted on the base 21 of the above-described opticalpickup 20 and is held in a fixed state thereon.

The semiconductor laser element 41 is a light-emitting element utilizingrecombination radiation of the semiconductor, and emits a laser beam(light beam) to be cast onto the signal recording surface of the opticaldisc 11.

The prism 42 has an inclined surface 42 a inclined at an inclination ofapproximately 45 degrees with respect to the board 46. This inclinedsurface 42 a reflects the light beam emitted from the semiconductorlaser element 41 into the direction substantially parallel to the board46 and bends the optical path of the light beam.

The optical member 43 is formed in a parallel-plate shape made of atransparent plastic material or glass and having a first surface 43 aand a second surface 43 b which are parallel to each other. This opticalmember 43 is joined onto the package member 45 so as to close theaperture 45 a of the package member 45 by the first surface 43 a.

On the first surface 43 a of the optical member 43, a grating 47 aslight beam splitting means is integrally formed at a position on theoptical path of the light beam which is reflected by the prism 42 andincident on the optical member 43.

This grating 47 is a diffraction grating for diffracting the lightincident thereon, and has a plurality of grooves extending in adirection slightly inclined with respect to the direction correspondingto the direction of radius of the optical disc 11. The grating 47 has afunction to split the light beam incident on the optical member 43 intoa plurality of beams including at least a main beam made up of a0th-order diffracted light and two side beams made up of plus and minus1st-order diffracted lights by the diffraction effect of these grooves.

Of the plurality of beams generated by splitting by the grating 47, themain beam is cast onto a predetermined recording track on the signalrecording surface of the optical disc 11. Of the plurality of beamsgenerated by splitting by the grating 47, the two side beams are cast topositions which are vertically away from the position irradiated withthe main beam along the recording track and which are horizontally awayfrom the center of the recording track by approximately ¼ track on thesignal recording surface of the optical disc 11. Thus, three light spotsare formed on the signal recording surface of the optical disc 11.Therefore, the optical pickup 20 is capable of detecting a trackingerror signal by a so-called three-beam method.

The size of the grating 47 is set so that a return light beam which isreflected by the signal recording surface of the optical disc 11, thenpassed through the objective lens and incident again on the opticalmember 43 and which is diffracted by a hologram 48, later described, anddirected toward the photodetector IC 44 does not become incident.

On the second surface 43 b of the optical member 43, the hologram 48 asoptical path branching means is integrally formed at a position on theoptical path of the return light beam which is reflected by the signalrecording surface of the optical disc 11, then passed through theobjective lens 27 and incident again on the optical member 43.

This hologram 48 directly transmits the light beam incident on theoptical member 43 from the first surface 43 a, that is, the light beamdirected toward the optical disc 11, and diffracts the light beamincident on the optical member 43 from the second surface 43 b, that is,the return light beam reflected by the signal recording surface of theoptical disc 11, into the direction toward the photodetector IC 44.

As the return light beam reflected by the signal recording surface ofthe optical disc 11 is diffracted by the hologram 48 into the directiontoward the photodetector IC 44, the optical path of the return lightbeam is separated from the optical path of the light beam incident onthe optical member 43 from the first surface 43 a.

This hologram 48 has two hologram areas 48 b and 48 c which are formedby bisecting along a boundary 48 a passing through the optical axis O ofthe return light beam of the main beam and in which holographic gratingsof different main diffraction angles are formed, respectively, as shownin FIG. 7. The holographic gratings of the hologram areas 48 b and 48 care constituted by a plurality of grooves extending in a substantiallyperpendicular direction with respect to the boundary 48 a. Therefore,the return light beam incident on the hologram 48 is split into twobeams by the boundary 48 a of the hologram 48, and a portion incident onthe one hologram area 48 b and a portion incident on the other hologramarea 48 c are diffracted at different diffraction angles into thedirection along the boundary 48 a. The diffracted beams are directed todifferent positions on the photodetector IC 44 as beams formingsubstantially semicircular spots as indicated by spots SP1 and SP2 inFIG. 7. In FIG. 7, only the main beam is shown.

In the optical pickup 20, as the individual portions of the return lightbeam generated by splitting by the boundary 48 a of the hologram 48 anddirected toward different positions on the photodetector IC 44 arereceived by the corresponding portions of the light-receiving section ofthe photodetector IC 44, a focusing error signal can be detected by theso-called Foucault method.

In the hologram 48, the boundary 48 a between the two hologram areas 48b and 48 c is inclined at a predetermined inclination with respect tothe direction corresponding to the direction of radius of the opticaldisc 11 so that the boundary does not coincide with the directioncorresponding to the direction of radius of the optical disc 11indicated by an arrow X in FIG. 7. Specifically, the boundary 48 abetween the two hologram areas 48 b and 48 c of the hologram 48 isinclined at approximately 45 degrees with respect to the directioncorresponding to the direction of radius of the optical disc 11.

In this case, the direction corresponding to the direction of radius ofthe optical disc 11 is a direction substantially equivalent to thedirection of radius of the optical disc 11 with respect to the returnlight beam incident on the hologram 48, and is a direction in which thereturn light beam incident on the hologram 48 moves when the spot of thelight beam cast onto the signal recording surface of the optical disc 11moves in the direction of radius of the optical disc 11.

In FIG. 7, to simplify the description, the rise mirror 32 is not shownwhich is arranged between the integrated optical element 40 and theobjective lens 27 so that the direction corresponding to the directionof radius of the optical disc 11 is the same as the direction of radiusof the optical disc 11. In FIG. 7, the semiconductor element 41, theprism 42, the optical member 43 and the photodetector IC 44 are shown asseparate members. Actually, however, these respective optical elementsare integrated as the integrated optical element 40, as described above.

As described above, in the case where the boundary 48 a between the twohologram areas 48 b and 48 c of the hologram 48 is not coincident withthe direction corresponding to the direction of radius of the opticaldisc 11, when the position of the objective lens 27 with respect to theoptical member 43, that is, the position of the objective lens 27 withrespect to the integrated optical element 40 and the base 21 of theoptical pickup 20 holding the integrated optical element, is deviatedinto the direction of radius of the optical disc 11 from the normalposition, the return light beam passed through the objective lens 27 andincident on the hologram 48 is asymmetrically split by the boundary 48 abetween the two hologram areas 48 b and 48 c of the hologram 48. Therespective parts of the return light beam generated by the asymmetricalsplitting are received by the corresponding portions of thelight-receiving section of the photodetector IC 44.

Thus, in the optical pickup 20, the position deviation of the objectivelens 27 in the direction of radius of the optical disc 11 can bedetected by finding the difference in the light quantity of therespective parts of the return light be am received by the correspondingportions of the light-receiving section of the photodetector IC 44.

In the above description, the boundary 48 a between the two hologramareas 48 b and 48 c of the hologram 48 is inclined at approximately 45degrees with respect to the direction corresponding to the direction ofradius of the optical disc 11. However, the angle of inclination of theboundary 48 a with respect to the direction corresponding to thedirection of radius of the optical disc 11 may be suitably set withinsuch a range that a sensitivity necessary for detecting the positiondeviation of the objective lens 27 in the direction of radius of theoptical disc 11 can be obtained.

The range which allows obtaining the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 is more or less dependent on theperformance of the individual members constituting the optical pickup20. In general, as long as the boundary 48 a is inclined at 15 degreesor more with respect to the direction corresponding to the direction ofradius of the optical disc 11, the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 can be obtained satisfactorily.

Therefore, it is desired that the boundary 48 a between the two hologramareas 48 b and 48 c of the hologram 48 is inclined at 15 degrees or morewith respect to the direction corresponding to the direction of radiusof the optical disc 11.

The photodetector IC 44 has a photodetector section for receiving thereturn light beam diffracted by the hologram 48 and transmitted throughthe optical member 43, and a voltage conversion circuit for converting acurrent from the photodetector section to a voltage. These section andcircuit are integrated as one element.

The photodetector section of the photodetector IC 44 has, as shown inFIG. 7, a center light-receiving section A, and two light-receivingsections B and C arranged at positions slightly distanced from thelight-receiving section A into the direction corresponding to thedirection along the recording track on the optical disc 11, as indicatedby an arrow Y in FIG. 7.

The direction corresponding to the direction along the recording trackon the optical disc 11 is a direction substantially equivalent to thedirection along the recording track on the optical disc 11 with respectto the return light beam incident on the photodetector section of thephotodetector IC 44. It is the direction in which the return light beamincident on the photodetector section moves when the spot of the lightbeam cast on the signal recording surface of the optical disc 11 movesalong the recording track on the optical disc 11.

Of these light-receiving sections of the photodetector section, thecenter light-receiving section A is divided by division lines which areparallel to the boundary 48 a between the two hologram areas 48 b and 48c of the hologram 48, that is, a division line d3 inclined atapproximately 45 degrees with respect to the direction corresponding tothe direction of radius of the optical disc 11 and a division line d4orthogonal to the division line d3, and thus has four light-receivingsections A1, A2, A3 and A4.

In the photodetector section of the photodetector IC 44, thelight-receiving section B and the light-receiving section C of theselight-receiving sections receive the return light beams of the two sidebeams generated by splitting by the grating 47. Also, in thephotodetector section of the photodetector IC 44, the light-receivingsections Al and A2 on one side of the division line d4 of the centerlight-receiving section A receive the part of the return light beam ofthe main beam which is diffracted by one hologram area 48 b of thehologram 48, and the light-receiving sections A3 and A4 on the otherside of the division line d4 of the center light-receiving section Areceive the part of the return light beam of the main beam which isdiffracted by the other hologram area 48 c of the hologram 48.

The light-receiving sections A1 and A2 for receiving the return lightdiffracted by one hologram area 48 b of the hologram 48, and thelight-receiving sections A3 and A4 for receiving the return lightdiffracted by the other hologram area 48 c of the hologram 48 aredivided by the division line d3 which is parallel to the boundary 48 abetween the two hologram areas 48 b and 48 c of the hologram 48.

In this optical pickup 20, as described above, the respective parts ofthe return light beam of the main beam generated by splitting by thehologram 48 are received by the two light-receiving sections separatedby the division line d3 which is parallel to the boundary 48 a of thehologram 48. Therefore, for example, even when the position on thephotodetector IC 44 of the return light beam spot of the light beam inthe focused state is slightly changed in the case where a change of theoscillation wavelength is generated in the semiconductor laser element41 or in the case where a change in the refractive index is generated inthe optical member 43 because of a temperature change, it is possible toeffectively restrain generation of any deviation of the signal level ofa focusing error signal due to the position change of the return lightbeam spot, and to detect an appropriate focusing error signal.

In the photodetector IC 44, the current values based on the lightquantities of the return lights received by the light-receiving sectionsA1, A2, A3, A4, B and C of the photodetector section are converted tovoltage values by the voltage conversion circuit and are supplied aslight-receiving signals to the signal processing circuit 15 of theoptical disc device 10. Then, the signal processing circuit 15 carriesout predetermined arithmetic processing based on the light-receivingsignals, thereby generating a reproduction signal RF1. Also, a focusingerror signal FE1 is generated by the so-called Foucault method and atracking error signal TR1 is generated by the so-called three-beammethod. Moreover, a signal indicating the position information of theobjective lens 27 is generated.

The operation for reproducing the signals recorded on the optical disc11 by using the optical disc device 10 constituted as described abovewill now be described.

In reproducing the signals recorded on the optical disc 11 by using theoptical disc device 10, first, the optical disc 11 is mounted on thespindle motor 12. Then, as the spindle motor 12 is rotationally drivenat a predetermined number of rotations under the control of the opticaldisc controller 14, the optical disc 11 is rotated.

A light beam is emitted from the semiconductor laser element 41 providedin the integrated optical element 40 of the optical pickup 20. Then, asthe thread motor of the head access control section 18 is driven underthe control of the optical disc controller 14, the optical pickup 20 ismoved at a high speed in the direction of radius of the optical disc 11so as to access a predetermined recording track on the signal recordingsurface of the optical disc 11.

The light beam emitted from the semiconductor laser element 41 isreflected by the inclined surface 42 a of the prism 42 and becomesincident on the optical member 43 from its first surface 43 a via theaperture 45 a of the package member 45. The light beam incident on theoptical member 43 is split into a plurality of beams including a mainbeam and two side beams by the grating 47 provided on the first surface43 a of the optical member 43, then transmitted through the opticalmember 43, and emitted from the integrated optical element 40.

The light beam emitted from the integrated optical element 40 isreflected by the reflection surface 32 a of the rise mirror 32 andbecomes incident on the objective lens 27. The light beam incident onthe objective lens 27 is converged by the objective lens 27 and thencast onto the predetermined recording track on the signal recordingsurface of the optical disc 11. In this case, three beam spots areformed on the signal recording surface of the optical disc 11 by themain beam and two side beams generated by splitting by the grating 47.

The light beam cast onto the signal recording surface of the opticaldisc 11 is reflected by the signal recording surface of the optical disc11 so as to be a return light beam including a signal component, whichis transmitted again through the objective lens 27 and becomes incidenton the optical member 43 from its second surface 43 b.

The return light beam incident on the optical member 43 from its secondsurface 43 b is diffracted by the hologram 48 formed on the secondsurface 43 b of the optical member 43, and the optical path of thisreturn light beam is thus separated from the optical path of the lightbeam incident on the optical member 43 from its first surface 43 a.

In this case, the return light beam is split into two parts by theboundary 48 a of the hologram 48. The part incident on one hologram area48 b and the part incident on the other hologram area 48 c arediffracted at different diffraction angles in the direction along theboundary 48 a and are directed toward different positions on thephotodetector IC 44.

The return light beams, diffracted by the hologram 48 and transmittedthrough the optical member 43, become incident on the package member 45via the aperture 45 a of the package member 45, then reach thephotodetector IC 44, and are received respectively by thelight-receiving sections A1, A2, A3, A4, B and C of the photodetectorsection of the photodetector IC 44.

Of the return light beams having reached the photodetector IC 44, returnlight beams of the two side beams generated by splitting by the grating47 are received respectively by the light-receiving section B and thelight-receiving section C of the photodetector section.

Of the return light beams having reached the photodetector IC 44, areturn light beam of the main beam which is diffracted by one hologramarea 48 b of the hologram 48 is received by the light-receiving sectionsA1 and A2 on one side of the division line d4 of the light-receivingsection A. Of the return light beams having reached the photodetector IC44, a return light beam of the main beam which is diffracted by theother hologram area 48 c of the hologram 48 is received by thelight-receiving sections A3 and A4 on the other side of the divisionline d4 of the light-receiving section A.

The return light beams received by the respective light-receivingsections A1, A2, A3, A4, B and C of the photodetector section aredetected as current values based on the light quantities by theselight-receiving sections A1, A2, A3, A4, B and C. The current valuesbased on the light quantities of the return light beams are converted tovoltage values by the voltage conversion circuit and are supplied aslight-receiving signals to the signal processing circuit 15.

Then, the signal processing circuit 15 carries out predeterminedarithmetic processing based on the light-receiving signals, therebygenerating a reproduction signal RF1. Also, a focusing error signal FE1is generated by the so-called Foucault method and a tracking errorsignal TR1 is generated by the so-called three-beam method.

In the optical disc device 10 to which the present invention is applied,as the signal processing circuit 15 carries out predetermined arithmeticprocessing based on the light-receiving signals, a signal S1 indicatingthe position information of the objective lens is generated.

The reproduction signal RF1 is found by the signal processing circuit 15carrying out arithmetic processing with respect to the followingequation (6), where SA1, SA2, SA3 and SA4 represent the light-receivingsignals based on the return light beam of the main beam received by thelight-receiving sections A1, A2, A3 and A4 of the photodetector sectionof the photodetector IC 44.

RF 1=(SA 1+SA 2)+(SA 3+SA 4)  (6)

The reproduction signal RF1 generated by the signal processing circuit15 is processed by error correction and then transmitted to an externalcomputer or the like via the interface 17. Thus, the external computeror the like can receive the signal recorded on the optical disc 11 asthe reproduction signal.

The focusing error signal FE1 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequations (7), (8) or (9), where SA1, SA2, SA3 and SA4 represent thelight-receiving signals based on the return light beam of the main beamreceived by the light-receiving sections A1, A2, A3 and A4 of thephotodetector section of the photodetector IC 44.

FE 1=SA 1−SA 2  (7)

FE 1=SA 3−SA 4  (8)

FE 1=(SA 1+SA 3)−(SA 2+SA 4)  (9)

The tracking error signal TR1 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequation (10), where SB and SC represent the light-receiving signalsbased on the return light beams of the two side beams received by thelight-receiving sections B and C of the photodetector section of thephotodetector IC 44.

TR 1=SB−SC  (10)

The focusing error signal FE1 and the tracking error signal TR1generated by the signal processing circuit 15 are supplied to the servocontrol section 19 via the optical disc controller 14.

On the basis of the focusing error signal FE1 and the tracking errorsignal TR1, the servo control section 19 drives the biaxial actuator 28of the optical pickup 20 under the control of the optical disccontroller 14.

As the biaxial actuator 28 is driven by the servo control section 19 inaccordance with the focusing error signal FE1 and the tracking errorsignal TR1, the biaxial actuator 28 carries out focusing servo tominutely move the objective lens 27 held by the lens holding section 31into the direction toward and away from the signal recording surface ofthe optical disc 11, thus controlling the focal point of the light beamconverged by the objective lens 27 so as to be constantly located on thesignal recording surface of the optical disc 11. The biaxial actuator 28also carries out tracking servo to minutely move the objective lens 27held by the lens holding section 31 into the direction of radius of theoptical disc 11, thus controlling the spot of the light beam convergedby the objective lens 27 so as to follow the recording track on theoptical disc 11.

By thus reading the reproduction signal while carrying out the focusingservo and tracking servo, the optical disc device 10 can appropriatelyreproduce the signals recorded on the optical disc 11 even in the casewhere the optical disc 11 is fluctuated or tilted.

Meanwhile, in the case where the signals recorded on the optical disc 11are to be reproduced by this optical disc device 10, first the opticalpickup 20 is moved at a high speed into the direction of radius of theoptical disc 11 mounted on the spindle motor 12 so as to access apredetermined recording track on the signal recording surface of theoptical disc 11, as described above. In this case, the tracking servo isoff.

The objective lens 27 of the optical pickup 20 is held by the lensholding section 31 movably supported to the fixed portion 29 of thebiaxial actuator 28 via the suspension 30, as described above.Therefore, when the optical pickup 20 is moved at a high speed into thedirection of radius of the. optical disc 11, or when this movement isstopped, the position of the objective lens 27 with respect to the fixedportion 29, that is, the position of the objective lens 27 with respectto the base 21 of the optical pickup 20 and the portion fixed thereto isdeviated into the direction of radius of the optical disc 11 from thenormal position, under the influence of inertia.

In the state where the objective lens 27 is deviated from the normalposition in this manner, appropriate recording/reproduction of signalsto/from the optical disc 11 cannot be carried out. The deviation of theobjective lens 27 from the normal position is eliminated with the lapseof a predetermined time period. However, if recording/reproduction iscarried out after the elimination of the deviation of the objective lens27, a relatively long time is required until the recording/reproductionof signals to/from a predetermined recording track on the optical disc11 is started, thus substantially obstructing the high-speed accessproperty.

Thus, in the optical disc device 10 to which the present invention isapplied, the position of the objective lens 27 is instantaneouslyrecovered to realize the high-speed access property, by detecting thedeviation of the objective lens 27 from the normal position as positioninformation, then causing the servo control section 19 to drive thebiaxial actuator 28 of the optical pickup 20 on the basis of theposition information of the objective lens 27, and then carrying outso-called midpoint servo for correcting the position deviation of theobjective lens 27.

In the optical disc device 10 to which the present invention is applied,when the position of the objective lens 27 with respect to the base 21of the optical pickup 20 and the portion fixed thereto is deviated fromthe normal position indicated in FIG. 8A into the direction of radius ofthe optical disc 11 indicated by an arrow X in FIGS. 8B and 8C, the spotof the return light beam incident on the optical member 43 of theintegrated optical element 40 from the second surface 43 b is deviatedon the hologram 48 into the direction corresponding to the direction ofradius of the optical disc 11.

When the objective lens 27 is at the normal position, the hologram 48 isdivided into the two hologram areas 48 b and 48 c by the boundary 48 apassing through the optical axis O of the return light beam of the mainbeam incident on the optical member 43, and the boundary 48 a betweenthe two hologram areas 48 b and 48 c is inclined at a predeterminedangle so as not to coincide with the direction corresponding to thedirection of radius of the optical disc 11.

Thus, when the position of the objective lens 27 with respect to thebase 21 of the optical pickup 20 and the portion fixed thereto isdeviated into the direction of radius of the optical disc 11 from thenormal position, the return light beam of the main beam incident on thehologram 48 is asymmetrically split by the boundary 48 a between the twohologram areas 48 b and 48 c of the hologram 48.

Specifically, when the objective lens 27 is at the normal position, thereturn light beam of the main beam becomes uniformly incident on the twohologram areas 48 b and 48 c of the hologram 48 and split into two partssubstantially uniformly by the boundary 48 a between the two hologramareas, as shown in FIG. 8A.

On the other hand, when the position of the objective lens 27 isdeviated into the direction of radius of the optical disc 11, the partof the return light beam of the main beam which is incident on onehologram area 48 b of the hologram 48 is smaller than the part incidenton the other hologram area 48 c, as shown in FIG. 8B, or the part of thereturn light beam of the main beam which is incident on the otherhologram area 48 c of the hologram 48 is smaller than the part incidenton the one hologram area 48 b, as shown in FIG. 8C. Thus, the returnlight beam of the main beam is asymmetrically split by the boundary 48 abetween the two hologram areas 48 b and 48 c.

Of the respective parts of the return light beam of the main beamasymmetrically split by the boundary 48 a between the two hologram areas48 b and 48 c of the hologram 48, the part diffracted by the onehologram area 48 b of the hologram 48 is received by the light-receivingsections A1 and A2 on one side of the division line d4 of thelight-receiving section A of the photodetector IC 44, and the partdiffracted by the other hologram area 48 c is received by thelight-receiving sections A3 and A4 on the other side of the divisionline d4 of the light-receiving section A.

Thus, when the position of the objective lens 27 is deviated into thedirection of radius of the optical disc 11 and the part of the returnlight beam of the main beam which is incident on the one hologram area48 b of the hologram 48 is smaller than the part incident on the otherhologram area 48 c, as shown in FIG. 8B, a dark spot is formed on thelight-receiving sections A1 and A2 on the one side of the division lined4 of the light-receiving section A of the photodetector IC 44, and abright spot is formed on the light-receiving sections A3 and A4 on theother side of the division line d4 of the light-receiving section A ofthe photodetector IC 44. Then, by finding the difference in the lightquantity between the light received by the light-receiving sections A1and A2 and the light received by the light-receiving sections A3 and A4,the position information of the objective lens 27 can be detected.

Meanwhile, when the position of the objective lens 27 is deviated intothe direction of radius of the optical disc 11 and the part of thereturn light beam of the main beam which is incident on the otherhologram area 48 c of the hologram 48 is smaller than the part incidenton the one hologram area 48 c, as shown in FIG. 8C, a dark spot isformed on the light-receiving sections A3 and A4 on the other side ofthe division line d4 of the light-receiving section A of thephotodetector IC 44, and a bright spot is formed on the light-receivingsections A1 and A2 on the one side of the division line d4 of thelight-receiving section A of the photodetector IC 44. Then, by findingthe difference in the light quantity between the light received by thelight-receiving sections A1 and A2 and the light received by thelight-receiving sections A3 and A4, the position information of theobjective lens 27 can be detected.

In the optical disc device 10 to which the present invention is applied,a signal indicating the position information of the objective lens 27 isgenerated by carrying out predetermined arithmetic processing by thesignal processing circuit 15 based on the light-receiving signals fromthe light-receiving sections A1, A2, A3 and A4 of the photodetectorsection.

The signal S1 indicating the position information of the objective lens27 is found by the signal processing circuit 15 carrying out arithmeticprocessing with respect to the following equation (11), where SA1, SA2,SA3 and SA4 represent the light-receiving signals based on the returnlight beam of the main beam received by the light-receiving sections A1,A2, A3 and A4, respectively, of the photodetector section of thephotodetector IC 44.

S 1=(SA 1+SA 2)−(SA 3+SA 4)  (11)

The signal S1 indicating the position information of the objective lens27, generated by the signal processing circuit 15, is supplied to theservo control section 19 via the optical disc controller 14.

Under the control of the optical disc controller 14, the servo controlsection 19 drives the biaxial actuator 28 of the optical pickup 20 onthe basis of the signal S1 indicating the position information of theobjective lens 27.

As the biaxial actuator 28 is driven by the servo control section 19 inresponse to the signal S1 indicating the position information of theobjective lens 27, the biaxial actuator 28 carries out midpoint servofor minutely moving the objective lens 27 held by the lens holdingsection 31 into the direction of radius of the optical disc 11 so as toinstantaneously correct the position deviation of the objective lens 27with respect to the base 21 of the optical pickup 20 and the portionsfixed thereto, and thus controls the objective lens 27 to be locatedconstantly at the normal position.

In the optical disc device 10 to which the present invention is applied,since the access operation of the optical pickup 20 is carried out whilethe midpoint servo is carried out as described above, the high-speedaccess property is realized.

Also, in the optical disc device 10 of this embodiment, since thefocusing error signal FE1 is detected by the so-called Foucault method,the spot of the return light beam formed on the light-receiving sectionof the photodetector IC 44 has a small diameter. Therefore, it ispossible to reduce the distance between the respective spots when themain beam and two side beams generated by splitting by the grating 47are cast onto the signal recording surface of the optical disc 11. Thus,the tolerance of the angle made by the direction of the recording trackon the optical disc 11 and the line connecting the respective spots canbe increased, and appropriate tracking servo and hence accurate signalrecording/reproduction can be carried out even in the case where anydefect such as scratch is generated on the optical disc 11.

Second Embodiment

Another embodiment of the optical disc device to which the presentinvention is applied will now be described. This optical disc device hasthe structure similar to that of the first embodiment except for theoptical member of the integrated optical element, which is slightlydifferent from that of the optical disc of the first embodiment.Therefore, the parts similar to those of the first embodiment aredenoted by the same numerals and will not be described further indetail. Only the parts different from those of the first embodiment willbe described in detail.

In the optical disc device 10 of the second embodiment, an opticalpickup 20 has an integrated optical element 50 as shown in FIG. 9. Theintegrated optical element 50 shown in FIG. 9 has the structure similarto that of the integrated optical element 40 of the first embodiment,except for an optical member 51 in place of the optical member 43 of theintegrated optical element 40 of the first embodiment.

The optical member 51 is formed in a parallel-plate shape made of atransparent plastic material or glass and having a first surface 51 aand a second surface 51 b which are parallel to each other. This opticalmember 51 is joined onto a package member 45 so as to close an aperture45 a of the package member 45 by the first surface 51 a.

On the first surface 51 a of the optical member 51, a grating 47 aslight beam splitting means is integrally formed at a position on theoptical path of the light beam which is reflected by a prism 42 andincident on the optical member 51.

This grating 47, similar to the grating 47 of the integrated opticalelement 40 of the first embodiment, has a plurality of grooves extendingin a direction slightly inclined with respect to the directioncorresponding to the direction of radius of the optical disc 11. Thegrating 47 has a function to split the light beam incident on theoptical member 51 into a plurality of beams including at least a mainbeam made up of a 0th-order diffracted light and two side beams made upof plus and minus 1st-order diffracted lights by the diffraction effectof these grooves.

On the second surface 51 b of the optical member 51, a hologram 52 asoptical path branching means is integrally formed at a position on theoptical path of the return light beam which is reflected by the signalrecording surface of the optical disc 11, then passed through anobjective lens 27 and incident again on the optical member 51.

Unlike the hologram 48 of the integrated optical element 40 of the firstembodiment, this hologram 52 as a whole is one hologram area. Thishologram area is made of a holographic grating constituted by aplurality of grooves extending in a direction inclined at approximately−45 degrees with respect to the direction corresponding to the directionof radius of the optical disc 11 indicated by an arrow X, as shown inFIG. 10.

This hologram 52 directly transmits the light beam incident on theoptical member 51 from the first surface 51 a, that is, the light beamdirected toward the optical disc 11, and diffracts the light beamincident on the optical member 51 from the second surface 51 b, that is,the return light beam reflected by the signal recording surface of theoptical disc 11, into the direction toward a photodetector IC 44.

As the return light beam reflected by the signal recording surface ofthe optical disc 11 is diffracted by the hologram 52 into the directiontoward the photodetector IC 44, the optical path of the return lightbeam is separated from the optical path of the light beam incident onthe optical member 51 from the first surface 51 a.

Also, on the first surface 51 a of the optical member 51, a Foucaultprism 53 as return light beam splitting means is integrally formed at aposition on the optical path of the return light beam diffracted by thehologram 52, as shown in FIGS. 10 and 11.

This Foucault prism 53 includes two surfaces 53 a and 53 b which havedifferent normal vectors. In this Foucault prism 53, a boundary 53 cbetween the two surfaces 53 a and 53 b is formed on the first surface 51a of the optical member 51 so as to pass through the center of thereturn light beam of the main beam diffracted by the hologram 52 and soas not to coincide with the direction corresponding to the direction ofradius of the optical disc 11, that is, so as to be inclined at apredetermined inclination with respect to the direction corresponding tothe direction of radius of the optical disc 11. Specifically, theboundary 53 c between the two surfaces 53 a and 53 b of the Foucaultprism 53 is inclined at approximately 45 degrees with respect to thedirection corresponding to the direction of radius of the optical disc11, as shown in FIG. 10.

In FIG. 10, it is assumed that the direction corresponding to thedirection of radius of the optical disc 11 is the same as the directionof radius of the optical disc 11.

Of the two surfaces 53 a and 53 b of the Foucault prism 53, the normalvector of the one surface 53 a forms the same angle as the diffractionangle of the hologram 52, with respect to the normal vector of the othersurface 53 b. Moreover, the normal vector of the other surface 53 b hasthe same component as the normal vector of the first surface 51 a of theoptical member 51.

The return light beam, diffracted by the hologram 52 and incident on theFoucault prism 53, is split into two beams by the boundary 53 c betweenthe two surfaces 53 a and 53 b of the Foucault prism 53, and a portionincident on the one surface 53 a and a portion incident on the othersurface 53 b travel into different directions and are directed towarddifferent positions on the photodetector IC 44.

Thus, in the optical pickup 20 having this integrated optical element50, as the individual portions of the return light beam generated bysplitting by the boundary 53 c between the two surfaces 53 a and 53 b ofthe Foucault prism 53 and directed toward different positions on thephotodetector IC 44 are received by the corresponding portions of thelight-receiving section of the photodetector IC 44, a focusing errorsignal can be detected by the so-called Foucault method.

Also, since the boundary 53 c between the two surfaces 53 a and 53 b ofthe Foucault prism 53 is formed so as not to coincide with the directioncorresponding to the direction of radius of the optical disc 11, theposition information of the objective lens 27 can be detected by findingthe difference in the light quantity of the return light beam split bythe boundary 53 c between the two surfaces 53 a and 53 b of the Foucaultprism 53.

That is, in the case where the Foucault prism 53 is constituted asdescribed above, when the position of the objective lens 27 with respectto the optical member 51, that is, the position of the objective lens 27with respect to the integrated optical element 50 and the base 21 of theoptical pickup 20 holding the integrated optical element, is deviatedinto the direction of radius of the optical disc 11 from the normalposition, the return light beam passed through the objective lens 27,then diffracted by the hologram 52 and incident on the Foucault prism 53is asymmetrically split by the boundary 53 c between the two surfaces 53a and 53 b of the Foucault prism 53. The respective parts of the returnlight beam generated by the asymmetrical splitting are received by thecorresponding portions of the light-receiving section of thephotodetector IC 44.

Therefore, in the optical pickup 20, the position deviation of theobjective lens 27 into the direction of radius of the optical disc 11can be detected by finding the difference in the light quantity of therespective parts of the return light beam received by the correspondingportions of the light-receiving section of the photodetector IC 44.

In the above description, the boundary 53 c between the two surfaces 53a and 53 b of the Foucault prism 53 is inclined at approximately 45degrees with respect to the direction corresponding to the direction ofradius of the optical disc 11. However, the angle of inclination of theboundary 53 c with respect to the direction corresponding to thedirection of radius of the optical disc 11 may be suitably set withinsuch a range that a sensitivity necessary for detecting the positiondeviation of the objective lens 27 in the direction of radius of theoptical disc 11 can be obtained.

The range which allows obtaining the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 is more or less dependent on theperformance of the individual members constituting the optical pickup20. In general, as long as the boundary 53 c is inclined at 15 degreesor more with respect to the direction corresponding to the direction ofradius of the optical disc 11, the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 can be obtained satisfactorily.

Therefore, it is desired that the boundary 53 c between the two surfaces53 a and 53 b of the Foucault prism 53 is inclined at 15 degrees or morewith respect to the direction corresponding to the direction of radiusof the optical disc 11.

The operation for reproducing the signals recorded on the optical disc11 by using the optical disc device 10 of the second embodimentconstituted as described above will now be described.

In reproducing the signals recorded on the optical disc 11 by using theoptical disc device 10, first, the optical disc 11 is mounted on thespindle motor 12. Then, as the spindle motor 12 is rotationally drivenat a predetermined number of rotations under the control of the opticaldisc controller 14, the optical disc 11 is rotated.

A light beam is emitted from the semiconductor laser element 41 providedin the integrated optical element 50 of the optical pickup 20. Then, asthe thread motor of the head access control section 18 is driven underthe control of the optical disc controller 14, the optical pickup 20 ismoved at a high speed in the direction of radius of the optical disc 11so as to access a predetermined recording track on the signal recordingsurface of the optical disc 11.

The light beam emitted from the semiconductor laser element 41 isreflected by the inclined surface 42 a of the prism 42 and becomesincident on the optical member 51 from its first surface 51 a via theaperture 45 a of the package member 45. The light beam incident on theoptical member 51 is split into a plurality of beams including a mainbeam and two side beams by the grating 47 provided on the first surface51 a of the optical member 51, then transmitted through the opticalmember 51, and emitted from the integrated optical element 50.

The light beam emitted from the integrated optical element 50 isreflected by the reflection surface 32 a of the rise mirror 32 andbecomes incident on the objective lens 27. The light beam incident onthe objective lens 27 is converged by the objective lens 27 and thencast onto the predetermined recording track on the signal recordingsurface of the optical disc 11. In this case, three beam spots areformed on the signal recording surface of the optical disc 11 by themain beam and two side beams generated by splitting by the grating 47.

The light beam cast onto the signal recording surface of the opticaldisc 11 is reflected by the signal recording surface of the optical disc11 so as to be a return light beam including a signal component, whichis transmitted again through the objective lens 27 and becomes incidenton the optical member 51 from its second surface 51 b.

The return light beam incident on the optical member 51 from its secondsurface 51 b is diffracted by the hologram 52 formed on the secondsurface 51 b of the optical member 51, and the optical path of thisreturn light beam is thus separated from the optical path of the lightbeam incident on the optical member 51 from its first surface 51 a.

The return light beam, diffracted by the hologram 52 formed on thesecond surface 51 b of the optical member 51, is transmitted through theoptical member 51 and is split into two beams by the Foucault prism 53formed on the first surface 51 a of the optical member 51. The partincident on the one surface 53 a of the Foucault prism 53 and the partincident on the other surface 53 b travel in different directions inaccordance with the normal vectors of the respective surfaces and aredirected toward different positions on the photodetector IC 44.

The two beams of the return light beam, generated by splitting by theFoucault prism 53, become incident on the package member 45 via theaperture 45 a of the package member 45, then reach the photodetector IC44, and are received respectively by the light-receiving sections A1,A2, A3, A4, B and C of the photodetector section of the photodetector IC44.

Of the return light beams having reached the photodetector IC 44, returnlight beams of the two side beams generated by splitting by the grating47 are received respectively by the light-receiving section B and thelight-receiving section C of the photodetector section.

Of the return light beams having reached the photodetector IC 44, areturn light beam of the main beam which is incident on the one surface53 a of the Foucault prism 53 a is received by the light-receivingsections A3 and A4 on one side of the division line d4 of thelight-receiving section A. Of the return light beams having reached thephotodetector IC 44, a return light beam of the main beam which isincident on the other surface 53 b of the Foucault prism 53 is receivedby the light-receiving sections A1 and A2 on the other side of thedivision line d4 of the light-receiving section A.

The return light beams received by the respective light-receivingsections A1, A2, A3, A4, B and C of the photodetector section aredetected as current values based on the light quantities by theselight-receiving sections A1, A2, A3, A4, B and C. The current valuesbased on the light quantities of the return light beams are converted tovoltage values by the voltage conversion circuit and are supplied aslight-receiving signals to the signal processing circuit 15.

Then, the signal processing circuit 15 carries out predeterminedarithmetic processing based on the light-receiving signals, therebygenerating a reproduction signal RF2. Also, a focusing error signal FE2is generated by the so-called Foucault method and a tracking errorsignal TR2 is generated by the so-called three-beam method.

In the optical disc device 10 of this embodiment, as the signalprocessing circuit 15 carries out predetermined arithmetic processingbased on the light-receiving signals, a signal S2 indicating theposition information of the objective lens is generated.

The reproduction signal RF2 is found by the signal processing circuit 15carrying out arithmetic processing with respect to the followingequation (12), where SA1, SA2, SA3 and SA4 represent the light-receivingsignals based on the return light beam of the main beam received by thelight-receiving sections A1, A2, A3 and A4 of the photodetector sectionof the photodetector IC 44.

 RF 2=(SA 1+SA 2)+(SA 3+SA 4)  (12)

The reproduction signal RF2 generated by the signal processing circuit15 is processed by error correction and then transmitted to an externalcomputer or the like via the interface 17. Thus, the external computeror the like can receive the signal recorded on the optical disc 11 asthe reproduction signal.

The focusing error signal FE2 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequations (13), (14) or (15), where SA1, SA2, SA3 and SA4 represent thelight-receiving signals based on the return light beam of the main beamreceived by the light-receiving sections A1, A2, A3 and A4 of thephotodetector section of the photodetector IC 44.

FE 2=SA 1−SA 2  (13)

FE 2=SA 3−SA 4  (14)

FE 2=(SA 1+SA 3)−(SA 2+SA 4)  (15)

The tracking error signal TR2 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequation (16), where SB and SC represent the light-receiving signalsbased on the return light beams of the two side beams received by thelight-receiving sections B and C of the photodetector section of thephotodetector IC 44.

TR 2=SB−SC  (16)

The focusing error signal FE2 and the tracking error signal TR2generated by the signal processing circuit 15 are supplied to the servocontrol section 19 via the optical disc controller 14.

On the basis of the focusing error signal FE2 and the tracking errorsignal TR2, the servo control section 19 drives the biaxial actuator 28of the optical pickup 20 under the control of the optical disccontroller 14.

As the biaxial actuator 28 is driven by the servo control section 19 inaccordance with the focusing error signal FE2 and the tracking errorsignal TR2, the biaxial actuator 28 carries out focusing servo tominutely move the objective lens 27 held by the lens holding section 31into the direction toward and away from the signal recording surface ofthe optical disc 11, thus controlling the focal point of the light beamconverged by the objective lens 27 so as to be constantly located on thesignal recording surface of the optical disc 11. The biaxial actuator 28also carries out tracking servo to minutely move the objective lens 27held by the lens holding section 31 into the direction of radius of theoptical disc 11, thus controlling the spot of the light beam convergedby the objective lens 27 so as to follow the recording track on theoptical disc 11.

By thus reading the reproduction signal while carrying out the focusingservo and tracking servo, the optical disc device 10 can appropriatelyreproduce the signals recorded on the optical disc 11 even in the casewhere the optical disc 11 is fluctuated or tilted.

Meanwhile, in the case where the signals recorded on the optical disc 11are to be reproduced by this optical disc device 10, first the opticalpickup 20 is moved at a high speed into the direction of radius of theoptical disc 11 mounted on the spindle motor 12 so as to access apredetermined recording track on the signal recording surface of theoptical disc 11, as described above. In this case, the tracking servo isoff.

The objective lens 27 of the optical pickup 20 is held by the lensholding section 31 movably supported to the fixed portion 29 of thebiaxial actuator 28 via the suspension 30, as described above.Therefore, when the optical pickup 20 is moved at a high speed into thedirection of radius of the optical disc 11, or when this movement isstopped, the position of the objective lens 27 with respect to the fixedportion 29, that is, the position of the objective lens 27 with respectto the base 21 of the optical pickup 20 and the portion fixed thereto isdeviated into the direction of radius of the optical disc 11 from thenormal position, under the influence of inertia.

Thus, in the optical disc device 10 to which the present invention isapplied, the position of the objective lens 27 is instantaneouslyrecovered to realize the high-speed access property, by detecting thedeviation of the objective lens 27 from the normal position as positioninformation, then causing the servo control section 19 to drive thebiaxial actuator 28 of the optical pickup 20 on the basis of theposition information of the objective lens 27, and then carrying outso-called midpoint servo for correcting the position deviation of theobjective lens 27.

In the optical disc device 10 to which the present invention is applied,when the position of the objective lens 27 with respect to the base 21of the optical pickup 20 and the portion fixed thereto is deviated fromthe normal position indicated in FIG. 12A into the direction of radiusof the optical disc 11 indicated by an arrow X in FIGS. 12B and 12C, thespot of the return light beam diffracted by the hologram 52 formed onthe second surface 51 b of the optical member 51 of the integratedoptical element 50 is deviated into the direction corresponding to thedirection of radius of the optical disc 11, on the Foucault prism 53formed on the first surface 51 a of the optical member 51.

When the objective lens 27 is at the normal position, the Foucault prism53 is divided into the two surfaces 53 a and 53 b by the boundary 53 cpassing through the center of the return light beam of the main beamdiffracted by the hologram 52, and the boundary 53 c between the twosurfaces 53 a and 53 b is inclined at a predetermined angle so as not tocoincide with the direction corresponding to the direction of radius ofthe optical disc 11.

Thus, when the position of the objective lens 27 with respect to thebase 21 of the optical pickup 20 and the portion fixed thereto isdeviated into the direction of radius of the optical disc 11 from thenormal position, the return light beam of the main beam incident on theFoucault prism 53 is asymmetrically split by the boundary 53 c betweenthe two surfaces 53 a and 53 b of the Foucault prism 53.

Specifically, when the objective lens 27 is at the normal position, thereturn light beam of the main beam becomes uniformly incident on the twosurfaces 53 a and 53 b of the Foucault prism 53 and split into two partssubstantially uniformly by the boundary 53 c between the two surfaces,as shown in FIG. 12A.

On the other hand, when the position of the objective lens 27 isdeviated into the direction of radius of the optical disc 11, the partof the return light beam of the main beam which is incident on the onesurface 53 a of the Foucault prism 53 is greater than the part incidenton the other surface 53 b, as shown in FIG. 12B, or the part of thereturn light beam of the main beam which is incident on the othersurface 53 b of the Foucault prism 53 is greater than the part incidenton the one surface 53 a, as shown in FIG. 12C. Thus, the return lightbeam of the main beam is asymmetrically split by the boundary 53 cbetween the two surfaces 53 a and 53 b.

Of the respective parts of the return light beam of the main beamasymmetrically split by the boundary 53 c between the two surfaces 53 aand 53 b of the Foucault prism 53, the part incident on the one surface53 a of the Foucault prism 53 is received by the light-receivingsections A3 and A4 on one side of the division line d4 of thelight-receiving section A of the photodetector IC 44, and the partincident on the other surface 53 b of the Foucault prism 53 is receivedby the light-receiving sections A1 and A2 on the other side of thedivision line d4 of the light-receiving section A.

Thus, when the position of the objective lens 27 is deviated into thedirection of radius of the optical disc 11 and the part of the returnlight beam of the main beam which is incident on the one surface 53 a ofthe Foucault prism 53 is greater than the part incident on the othersurface 53 b, as shown in FIG. 12B, a dark spot is formed on thelight-receiving sections A1 and A2 on the one side of the division lined4 of the light-receiving section A of the photodetector IC 44, and abright spot is formed on the light-receiving sections A3 and A4 on theother side of the division line d4 of the light-receiving section A ofthe photodetector IC 44. Then, by finding the difference in the lightquantity between the light received by the light-receiving sections A1and A2 and the light received by the light-receiving sections A3 and A4,the position information of the objective lens 27 can be detected.

Meanwhile, when the position of the objective lens 27 is deviated intothe direction of radius of the optical disc 11 and the part of thereturn light beam of the main beam which is incident on the othersurface 53 b of the Foucault prism 53 is greater than the part incidenton the one surface 53 a, as shown in FIG. 12C, a dark spot is formed onthe light-receiving sections A3 and A4 on the other side of the divisionline d4 of the light-receiving section A of the photodetector IC 44, anda bright spot is formed on the light-receiving sections A1 and A2 on theone side of the division line d4 of the light-receiving section A of thephotodetector IC 44. Then, by finding the difference in the lightquantity between the light received by the light-receiving sections A1and A2 and the light received by the light-receiving sections A3 and A4,the position information of the objective lens 27 can be detected.

In the optical disc device 10 of this embodiment, a signal indicatingthe position information of the objective lens 27 is generated bycarrying out predetermined arithmetic processing by the signalprocessing circuit 15 based on the light-receiving signals from thelight-receiving sections A1, A2, A3 and A4 of the photodetector section.

The signal S2 indicating the position information of the objective lens27 is found by the signal processing circuit 15 carrying out arithmeticprocessing with respect to the following equation (17), where SA1, SA2,SA3 and SA4 represent the light-receiving signals based on the returnlight beam of the main beam received by the light-receiving sections A1,A2, A3 and A4, respectively, of the photodetector section of thephotodetector IC 44.

S 2=(SA 1+SA 2)−(SA 3+SA 4)  (17)

The signal S2 indicating the position information of the objective lens27, generated by the signal processing circuit 15, is supplied to theservo control section 19 via the optical disc controller 14.

Under the control of the optical disc controller 14, the servo controlsection 19 drives the biaxial actuator 28 of the optical pickup 20 onthe basis of the signal S2 indicating the position information of theobjective lens 27.

As the biaxial actuator 28 is driven by the servo control section 19 inresponse to the signal S2 indicating the position information of theobjective lens 27, the biaxial actuator 28 carries out midpoint servofor minutely moving the objective lens 27 held by the lens holdingsection 31 into the direction of radius of the optical disc 11 so as toinstantaneously correct the position deviation of the objective lens 27with respect to the base 21 of the optical pickup 20 and the portionsfixed thereto, and thus controls the objective lens 27 to be locatedconstantly at the normal position.

In the optical disc device 10 of this embodiment, since the accessoperation of the optical pickup 20 is carried out while the midpointservo is carried out as described above, the high-speed access propertyis realized.

Also, in the optical disc device 10 of this embodiment, since thefocusing error signal FE2 is detected by the so-called Foucault method,the spot of the return light beam formed on the light-receiving sectionof the photodetector IC 44 has a small diameter. Therefore, it ispossible to reduce the distance between the respective spots when themain beam and two side beams generated by splitting by the grating 47are cast onto the signal recording surface of the optical disc 11. Thus,the tolerance of the angle made by the direction of the recording trackon the optical disc 11 and the line connecting the respective spots canbe increased, and appropriate tracking servo and hence accurate signalrecording/reproduction can be carried out even in the case where anydefect such as scratch is generated on the optical disc 11.

Third Embodiment

Still another embodiment of the optical disc device to which the presentinvention is applied will now be described. This optical disc device hasthe basic structure similar to that of the optical disc device 10 of thefirst embodiment and is characterized in that it is adapted to detect atracking error signal by a so-called push-pull method.

Therefore, in the following description, only this characteristic pointwill be described. The parts similar to those of the first embodimentare denoted by the same numerals and will not be described further indetail.

In the optical disc device 10 of the third embodiment, an optical pickup20 has an integrated optical element 60 as shown in FIG. 13. Theintegrated optical element 60 shown in FIG. 13 has the structure similarto that of the integrated optical element 40 of the first embodiment,except for an optical member 61 in place of the optical member 43 of theintegrated optical element 40 of the first embodiment and for aphotodetector IC 62 in place of the photodetector IC 44 of theintegrated optical element 40 of the first embodiment.

The optical member 61 is formed in a parallel-plate shape made of atransparent plastic material or glass and having a first surface 61 aand a second surface 61 b which are parallel to each other. This opticalmember 61 is joined onto the package member 45 so as to close theaperture 45 a of the package member 45 by the first surface 6 a.

On the first surface 61 a of the optical member 61, there is formed nograting 47 such as the one integrally formed on the first surface 43 aof the optical member 43 of the first embodiment is not formed.Therefore, a light beam incident on the optical member 61 from its firstsurface 61 a is transmitted through the optical member 61 without beingsplit into a plurality of beams.

On the second surface 61 b of the optical member 61, a hologram 48similar to the one formed on the second surface 43 b of the opticalmember 43 of the first embodiment is integrally formed.

This hologram 48 directly transmits the light beam incident on theoptical member 61 from the first surface 61 a, that is, the light beamdirected toward the optical disc 11, and diffracts the light beamincident on the optical member 61 from the second surface 61 b, that is,the return light beam reflected by the signal recording surface of theoptical disc 11, into the direction toward the photodetector IC 62.

As the return light beam reflected by the signal recording surface ofthe optical disc 11 is diffracted by the hologram 48 into the directiontoward the photodetector IC 62, the optical path of the return lightbeam is separated from the optical path of the light beam incident onthe optical member 61 from the first surface 61 a.

This hologram 48 is bisected along a boundary 48 a passing through theoptical axis O of the return light beam of the main beam and thus hastwo hologram areas 48 b and 48 c in which holographic gratings ofdifferent main diffraction angles are formed, respectively, as shown inFIG. 14. The holographic gratings of the two hologram areas 48 b and 48c are constituted by a plurality of grooves extending in a substantiallyperpendicular direction with respect to the boundary 48 a. Therefore,the return light beam incident on the hologram 48 is split into twobeams by the boundary 48 a of the hologram 48, and a portion incident onthe one hologram area 48 b and a portion incident on the other hologramarea 48 c are diffracted at different diffraction angles into thedirection along the boundary 48 a. The diffracted beams are directed todifferent positions on the photodetector IC 62 as beams formingsubstantially semicircular spots as indicated by spots SP3 and SP4 inFIG. 14.

In the optical pickup 20, as the individual portions of the return lightbeam generated by splitting by the boundary 48 a of the hologram 48 anddirected toward different positions on the photodetector IC 62 arereceived by the corresponding portions of the light-receiving section ofthe photodetector IC 62, a focusing error signal can be detected by theso-called Foucault method.

In the hologram 48, the boundary 48 a between the two hologram areas 48b and 48 c is inclined at a predetermined inclination with respect tothe direction corresponding to the direction of radius of the opticaldisc 11 so that the boundary does not coincide with the directioncorresponding to the direction of radius of the optical disc 11indicated by an arrow X in FIG. 14. Specifically, the boundary 48 abetween the two hologram areas 48 b and 48 c of the hologram 48 isinclined at approximately 45 degrees with respect to the directioncorresponding to the direction of radius of the optical disc 11.

In FIG. 14, to simplify the description, the direction corresponding tothe direction of radius of the optical disc 11 is the same as thedirection of radius of the optical disc 11.

As described above, in the case where the boundary 48 a between the twohologram areas 48 b and 48 c of the hologram 48 is not coincident withthe direction corresponding to the direction of radius of the opticaldisc 11, when the position of the objective lens 27 with respect to theoptical member 61, that is, the position of the objective lens 27 withrespect to the integrated optical element 40 and the base 21 of theoptical pickup 20 holding the integrated optical element, is deviatedinto the direction of radius of the optical disc 11 from the normalposition, the return light beam passed through the objective lens 27 andincident on the hologram 48 is asymmetrically split by the boundary 48 abetween the two hologram areas 48 b and 48 c of the hologram 48. Therespective parts of the return light beam generated by the asymmetricalsplitting are received by the corresponding portions of thelight-receiving section of the photodetector IC 62.

Thus, in the optical pickup 20, the position deviation of the objectivelens 27 in the direction of radius of the optical disc 11 can bedetected by finding the difference in the light quantity of therespective parts of the return light beam received by the correspondingportions of the light-receiving section of the photodetector IC 62.

In the above description, the boundary 48 a between the two hologramareas 48 b and 48 c of the hologram 48 is inclined at approximately 45degrees with respect to the direction corresponding to the direction ofradius of the optical disc 11. However, the angle of inclination of theboundary 48 a with respect to the direction corresponding to thedirection of radius of the optical disc 11 may be suitably set withinsuch a range that a sensitivity necessary for detecting the positiondeviation of the objective lens 27 in the direction of radius of theoptical disc 11 can be obtained.

The range which allows obtaining the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 is more or less dependent on theperformance of the individual members constituting the optical pickup20. In general, as long as the boundary 48 a is inclined at 15 degreesor more with respect to the direction corresponding to the direction ofradius of the optical disc 11, the sensitivity necessary for detectingthe position deviation of the objective lens 27 in the direction ofradius of the optical disc 11 can be obtained satisfactorily.

Therefore, it is desired that the boundary 48 a between the two hologramareas 48 b and 48 c of the hologram 48 is inclined at 15 degrees or morewith respect to the direction corresponding to the direction of radiusof the optical disc 11.

The photodetector IC 62 has a photodetector section for receiving thereturn light beam diffracted by the hologram 48 and transmitted throughthe optical member 61, and a voltage conversion circuit for converting acurrent from the photodetector section to a voltage. These section andcircuit are integrated as one element.

The photodetector section of the photodetector IC 61 has, as shown inFIG. 14, four light-receiving sections D, E, F and G arranged bydivision lines which are parallel to the boundary 48 a between the twohologram areas 48 b and 48 c of the hologram 48, that is, a divisionline d5 inclined at approximately 45 degrees with respect to thedirection corresponding to the direction of radius of the optical disc11 and a division line d6 orthogonal to the division line d5.

In the photodetector section of the photodetector IC 62, thelight-receiving sections D and E of these light-receiving sections onone side of the division line d6 receive a part of the return light beamwhich is diffracted by the one hologram area 48 b of the hologram 48,and the light-receiving sections F and G on the other side of thedivision line d6 receive a part of the return light beam which isdiffracted by the other hologram area 48 c of the hologram 48.

In the photodetector IC 62, the current values based on the lightquantities of the return lights received by the light-receiving sectionsD, E, F and G of the photodetector section are converted to voltagevalues by the voltage conversion circuit and are supplied aslight-receiving signals to the signal processing circuit 15 of theoptical disc device 10. Then, the signal processing circuit 15 carriesout predetermined arithmetic processing based on the light-receivingsignals, thereby generating a reproduction signal RF3. Also, a focusingerror signal FE3 is generated by the so-called Foucault method and atracking error signal TR3 is generated by the so-called push-pullmethod. Moreover, a signal S3 indicating the position information of theobjective lens 27 is generated.

The reproduction signal RF3 is found by the signal processing circuit 15carrying out arithmetic processing with respect to the followingequation (18), where SD, SE, SF and SG represent the light-receivingsignals based on the return light beam of the main beam received by thelight-receiving sections D, E, F and G of the photodetector section ofthe photodetector IC 62.

RF 3=(SD+SE)+(SF+SG)  (18)

The reproduction signal RF3 generated by the signal processing circuit15 is processed by error correction and then transmitted to an externalcomputer or the like via the interface 17. Thus, the external computeror the like can receive the signal recorded on the optical disc 11 asthe reproduction signal.

The focusing error signal FE3 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequations (19), (20) or (21).

FE 3=SD−SE  (19)

FE 3=SF−SG  (20)

FE 3=(SD+SF)−(SE+SG)  (21)

The tracking error signal TR3 is found by the signal processing circuit15 carrying out arithmetic processing with respect to the followingequation (22).

TR 3=(SD+SE)−(SF+SG)  (22)

The focusing error signal FE3 and the tracking error signal TR3generated by the signal processing circuit 15 are supplied to the servocontrol section 19 via the optical disc controller 14.

On the basis of the focusing error signal FE3 and the tracking errorsignal TR3, the servo control section 19 drives the biaxial actuator 28of the optical pickup 20 so as to carry out focusing servo and trackingservo under the control of the optical disc controller 14.

By thus reading the reproduction signal while carrying out the focusingservo and tracking servo, the optical disc device 10 can appropriatelyreproduce the signals recorded on the optical disc 11 even in the casewhere the optical disc 11 is fluctuated or tilted.

The signal S3 indicating the position information of the objective lens27 is found by the signal processing circuit 15 carrying out arithmeticprocessing with respect to the following equation (23).

S 3=(SD+SE)−(SF+SG)  (23)

The signal S3 indicating the position information of the objective lens27 generated by the signal processing circuit 15 is supplied to theservo control section 19 via the optical disc controller 14.

Under the control of the optical disc controller 14, the servo controlsection 19 drives the biaxial actuator 28 of the optical pickup 20 onthe basis of the signal S3 indicating the position information of theobjective lens 27.

As the biaxial actuator 28 is driven by the servo control section 19 inresponse to the signal S3 indicating the position information of theobjective lens 27, the biaxial actuator 28 carries out midpoint servofor minutely moving the objective lens 27 held by the lens holdingsection 31 into the direction of radius of the optical disc 11 so as toinstantaneously correct the position deviation of the objective lens 27with respect to the base 21 of the optical pickup 20 and the portionsfixed thereto, and thus controls the objective lens 27 to be locatedconstantly at the normal position.

In the optical disc device 10 of this embodiment, since the accessoperation of the optical pickup 20 is carried out while the midpointservo is carried out as described above, the high-speed access propertyis realized.

Also, in the optical disc device 10 of this embodiment, since thetracking error signal TR3 can be detected by the so-called push-pullmethod, the grating 47 for splitting a light beam and thelight-receiving sections for receiving side beams split by the grating47 are no longer necessary, thus enabling simplification of thestructure.

As described above, in the optical disc device to which the presentinvention is applied, the position deviation of the objective lens 27 atthe time of access operation or the like of the optical pickup 20 isdetected, and the midpoint servo is carried out for minutely moving theobjective lens 27 in the direction of radius of the optical disc 11 soas to instantaneously correct the position deviation of the objectivelens 27 with respect to the base 21 of the optical pickup 20 and theportions fixed thereto. Therefore, the accuracy and stability of theaccess operation of the optical pickup 20 can be secured and thehigh-speed access property can be realized.

Also, in the optical disc device 10 to which the present invention isapplied, the position deviation of the objective lens 27 is detected byusing the hologram 48 or the Foucault prism 62 of the optical pickup 20for generating a focusing error signal by the Foucault method, withoutseparately providing any means for detecting the position deviation ofthe objective lens 27. Therefore, the position deviation of theobjective lens can be detected appropriately and simply, and thehigh-speed access property can be realized without causing the increasein the number of component parts, the increase in size of the deviceitself and the rise in cost.

Moreover, in the optical disc device 10 to which the present inventionis applied, the center light-receiving section of the photodetector IC44, 62 is divided by the division line d3, d5 parallel to the boundary48 a of the hologram 48 or the boundary 53 a of the Foucault prism 53,and the respective parts of the return light beam of the main beam splitby the hologram 48 are received by the two light-receiving sectionsgenerated by division by the division line d3, d5 parallel to theboundary 48 a of the hologram 48 or the boundary 53 a of the Foucaultprism 53. Therefore, for example, even when the position on thephotodetector IC 44, 62 of the return light beam spot of the light beamin the focused state is slightly changed in the case where a change ofthe oscillation wavelength is generated in the semiconductor laserelement 41 or in the case where a change in the refractive index isgenerated in the optical member 43, 51, 61 because of a temperaturechange, it is possible to effectively restrain generation of anydeviation of the signal level of a focusing error signal due to theposition change of the return light beam spot, and to detect anappropriate focusing error signal.

Also, in the optical disc device 10 to which the present invention isapplied, the grating 47 as the light beam splitting means, the hologram48, 52 as the optical path branching means, and the Foucault prism 53 asthe return light beam splitting means are integrally formed in theoptical member 43, 51, 61. Therefore, alignment of these members is notnecessary. Thus, it is possible to simplify the operation of assemblingthe optical pickup 20 and to reduce the number of component parts andthe cost.

The optical member 43, 51, 61 can be easily prepared byinjection-molding of a resin material or by glass press molding.

In the optical disc device 10 to which the present invention is applied,the semiconductor laser element 41 of the optical pickup 20, thephotodetector IC 44, 62, and the optical member 43, 51, 61 areconstituted as the integrated optical element 40, 50, 60. Therefore,alignment of these members is not necessary. Thus, it is possible tofurther simplify the operation of assembling the optical pickup 20 andto further reduce the number of component parts and the cost. Also,reduction in size and thickness of the overall device can be realized.

In the above description, the optical disc device has an optical pickupof a non-polarization optical system for recording/reproducing signalsto/from a compact disc (CD) or a CD-ROM. However, this invention is notlimited to the above-described embodiments and can also be applied anoptical disc device having an optical pickup of a polarization opticalsystem for recording/reproducing signals to/from a magneto-optical disc(MO) or the like.

Industrial Applicability

According to the integrated optical element of the present invention,since the position deviation of the light beam converging means withrespect to the integrated optical element at the time of access to adesired recording track on the optical disc can be detected, the opticalpickup using this integrated optical element can realize the high-speedaccess property.

Also, in this integrated optical element, the optical path branchingmeans or the return light beam splitting means is used for detecting theposition deviation of the light beam converging means with respect tothe integrated optical element, instead of additionally providing anymeans for detecting the position deviation of the light beam convergingmeans. Therefore, the optical pickup using this integrated opticalelement can realize the high-speed access property by appropriately andsimply detecting the position deviation of the light beam convergingmeans, without causing the increase in the number of components, theincrease in the size of the device itself and the rise in cost.

According to the optical pickup of the present invention, since theposition deviation of the light beam converging means with respect tothe optical member at the time of access to a desired recording track onthe optical disc can be detected, the position of the light beamconverging means can be instantaneously recovered to secure the accuracyand stability of the access operation and to realize the high-speedaccess property.

Also, in this optical pickup according to the present invention, theoptical path branching means or the return light beam splitting means isused for detecting the position deviation of the light beam convergingmeans with respect to the optical member, instead of additionallyproviding any means for detecting the position deviation of the lightbeam converging means. Therefore, the optical pickup can realize thehigh-speed access property by appropriately and simply detecting theposition deviation of the light beam converging means, without causingthe increase in the number of components, the increase in the size ofthe device itself and the rise in cost.

According to the optical disc device of the present invention, since theposition deviation of the light beam converging means with respect tothe optical member at the time when causing the optical pickup to accessa desired recording track on the optical disc can be detected, theposition of the light beam converging means can be instantaneouslyrecovered to secure the accuracy and stability of the access operationand to realize the high-speed access property.

Also, in this optical disc device according to the present invention,the optical path branching means or the return light beam splittingmeans of the optical pickup is used for detecting the position deviationof the light beam converging means with respect to the optical member,instead of additionally providing any means for detecting the positiondeviation of the light beam converging means. Therefore, the opticaldisc device can realize the high-speed access property by appropriatelyand simply detecting the position deviation of the light beam convergingmeans, without causing the increase in the number of components, theincrease in the size of the device itself and the rise in cost.

What is claimed is:
 1. An integrated optical element used for an optical pickup for carrying out recording and/or reproduction of signals by irradiating a signal recording surface of an optical disc with a light beam, comprising: a light source for emitting the light beam; a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc; a package member for housing the light source and the photodetector therein; an optical member arranged on the package member for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector; and optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam directed toward the photodetector; the optical path branching means having at least two diffraction areas for diffracting the return light beam reflected by the signal recording surface of the optical disc, into different directions, respectively, the boundary between the diffraction areas being inclined at a predetermined angle with respect to the direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving a return light beam diffracted by one diffraction area of the optical path branching means and a portion for receiving a return light beam diffracted by the other diffraction area.
 2. The integrated optical element as claimed in claim 1, wherein the optical path branching means is made of a hologram formed on the surface of optical member.
 3. The integrated optical element as claimed in claim 2, wherein the optical member has a first surface on which the light beam emitted from the light source is incident and a second surface which is substantially parallel to the first surface and on which the return light beam reflected by the signal recording surface of the optical disc is incident, and wherein the hologram as the optical path branching means is formed on the second surface.
 4. The integrated optical element as claimed in claim 1, wherein light beam splitting means for diffracting the light beam directed toward the optical disc and for splitting the light beam into a plurality of beams including a main beam and two side beams is formed integrally with the optical member.
 5. The integrated optical element as claimed in claim 4, wherein light beam splitting means for diffracting the light beam directed toward the optical disc and for splitting the light beam into a plurality of beams including a main beam and two side beams is formed on the first surface of the optical member.
 6. The integrated optical element as claimed in claim 1, wherein the optical path branching means is formed so that the boundary between the diffraction areas is inclined at 15 degrees or more with respect to the direction corresponding to the direction of radius of the optical disc.
 7. The integrated optical element as claimed in claim 1, wherein at least one light-receiving section of the photodetector is divided by a division line substantially parallel to the boundary between the diffraction areas of the optical path branching means.
 8. An integrated optical element used for an optical pickup for carrying out recording and/or reproduction of signals by irradiating a signal recording surface of an optical disc with a light beam, comprising: a light source for emitting the light beam; a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc; a package member for housing the light source and the photodetector therein; an optical member arranged on the package member for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector; optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam reflected by the signal recording surface of the optical disc; and return light beam splitting means integrally formed with the optical member for splitting the return light beam passed through the optical path branching means into at least two beams; the return light beam splitting means having at least two surfaces having different normal vectors, the boundary between these surfaces being inclined at a predetermined angle with respect to a direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving one return light beam generated by the return light beam splitting means and a portion for receiving the other return light beam.
 9. The integrated optical element as claimed in claim 8, wherein the optical path branching means is made of a hologram formed on a surface of optical member.
 10. The integrated optical element as claimed in claim 9, wherein the optical member has a first surface on which the light beam emitted from the light source is incident and a second surface which is substantially parallel to the first surface and on which the return light beam reflected by the signal recording surface of the optical disc is incident, and wherein the return light beam splitting means is formed on the first surface and the hologram as the optical path branching means is formed on the second surface.
 11. The integrated optical element as claimed in claim 8, wherein light beam splitting means for diffracting the light beam directed toward the optical disc and for splitting the light beam into a plurality of beams including a main beam and two side beams is formed integrally with the optical member.
 12. The integrated optical element as claimed in claim 11, wherein light beam splitting means for diffracting the light beam directed toward the optical disc and for splitting the light beam into a plurality of beams including a main beam and two side beams is formed on the first surface of the optical member.
 13. The integrated optical element as claimed in claim 8, wherein the optical path branching means is formed so that the boundary is inclined at 15 degrees or more with respect to the direction corresponding to the direction of radius of the optical disc.
 14. The integrated optical element as claimed in claim 8, wherein at least one light-receiving section of the photodetector is divided by a division line substantially parallel to the boundary of the return light beam splitting means.
 15. An optical pickup for carrying out recording and/or reproduction of signals by irradiating a signal recording surface of an optical disc with a light beam, comprising: a light source for emitting the light beam; light beam converging means for converging the light beam emitted from the light source and for irradiating the signal recording surface of the optical disc with the converged light beam; a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc; an optical member arranged between the light source and photodetector on one side and the light beam converging means on the other side, for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector; optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam directed toward the photodetector; and a biaxial actuator for moving the light beam converging means into biaxial directions, that is, the direction of radius of the optical disc and the direction toward and away from the optical disc; the optical path branching means having at least two diffraction areas for diffracting the return light beam reflected by the signal recording surface of the optical disc, into different directions, respectively, the boundary between the diffraction areas being inclined at a predetermined angle with respect to the direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving a return light beam diffracted by one diffraction area of the optical path branching means and a portion for receiving a return light beam diffracted by the other diffraction area.
 16. An optical pickup for carrying out recording and/or reproduction of signals by irradiating a signal recording surface of an optical disc with a light beam, comprising: a light source for emitting the light beam; light beam converging means for converging the light beam emitted from the light source and for irradiating the signal recording surface of the optical disc with the converged light beam; a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc; an optical member arranged between the light source and photodetector on one side and the light beam converging means on the other side, for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector; optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam reflected by the signal recording surface of the optical disc; return light beam splitting means integrally formed with the optical member for splitting the return light beam passed through the optical path branching means into at least two beams; and a biaxial actuator for moving the light beam converging means into biaxial directions, that is, the direction of radius of the optical disc and the direction toward and away from the optical disc; the return light beam splitting means having at least two surfaces having different normal vectors, the boundary between these surfaces being inclined at a predetermined angle with respect to a direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving one return light beam generated by the return light beam splitting means and a portion for receiving the other return light beam.
 17. An optical disc device comprising: disc rotating means for rotating an optical disc; an optical pickup for carrying out recording and/or reproduction of signals by irradiating with a light beam a signal recording surface of the optical disc rotated by the disc rotating means; a signal processing circuit for processing a detection signal from the optical pickup; and an access mechanism for moving the optical pickup in the direction of radius of the optical disc; the optical pickup comprising a light source for emitting the light beam, light beam converging means for converging the light beam emitted from the light source and for irradiating the signal recording surface of the optical disc with the converged light beam, a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc, an optical member arranged between the light source and photodetector on one side and the light beam converging means on the other side, for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector, optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam directed toward the photodetector, and a biaxial actuator for moving the light beam converging means into biaxial directions, that is, the direction of radius of the optical disc and the direction toward and away from the optical disc; the optical path branching means having at least two diffraction areas for diffracting the return light beam reflected by the signal recording surface of the optical disc, into different directions, respectively, the boundary between the diffraction areas being inclined at a predetermined angle with respect to the direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving a return light beam diffracted by one diffraction area of the optical path branching means and a portion for receiving a return light beam diffracted by the other diffraction area.
 18. An optical disc device comprising: disc rotating means for rotating an optical disc; an optical pickup for carrying out recording and/or reproduction of signals by irradiating with a light beam a signal recording surface of the optical disc rotated by the disc rotating means; a signal processing circuit for processing a detection signal from the optical pickup; and an access mechanism for moving the optical pickup in the direction of radius of the optical disc; the optical pickup comprising a light source for emitting the light beam, light beam converging means for converging the light beam emitted from the light source and for irradiating the signal recording surface of the optical disc with the converged light beam, a photodetector having a light-receiving section for receiving a return light beam reflected by the signal recording surface of the optical disc, an optical member arranged between the light source and photodetector on one side and the light beam converging means on the other side, for transmitting the light beam emitted from the light source and for transmitting the return light beam directed toward the photodetector, optical path branching means integrally formed with the optical member for separating the optical path of the light beam emitted from the light source and the optical path of the return light beam reflected by the signal recording surface of the optical disc, return light beam splitting means integrally formed with the optical member for splitting the return light beam passed through the optical path branching means into at least two beams, and a biaxial actuator for moving the light beam converging means into biaxial directions, that is, the direction of radius of the optical disc and the direction toward and away from the optical disc; the return light beam splitting means having at least two surfaces having different normal vectors, the boundary between these surfaces being inclined at a predetermined angle with respect to a direction corresponding to the direction of radius of the optical disc; at least one light-receiving section of the photodetector being divided into a portion for receiving one return light beam generated by the return light beam splitting means and a portion for receiving the other return light beam. 